When Kim Jacobs was preparing to get married last month for the second time, she decided it was time to indulge herself with a truly grand engagement ring.

She walked out of Mervis Diamond Importers at Tysons Corner with a shimmering ring -- a 1.27-carat center stone flanked by smaller diamonds -- plus a diamond necklace for her wedding day.

"I wanted to get exactly what I wanted," said Jacobs, the 40-year-old co-owner of a Springfield office furniture store. "I feel good about my economic position. Business is good overall."

A lot of people seem to have that frame of mind nowadays in Fairfax County, which ranked as the wealthiest of more than 3,000 U.S. jurisdictions in a recent survey.

Flush with the financial fallout from Northern Virginia's technology boom, Fairfax County has widened its economic lead over Montgomery County. Much of the county's newest wealth, moreover, is in the hands of young Internet millionaires eager to enjoy the fruits of almost overnight stock windfalls.

"Fairfax is just white hot," said diamond importer Ronnie Mervis, whose voice is heard on countless radio ads for the firm he and two brothers own. "There's so much growth, go, go, go."

Last year, the firm moved its headquarters from Rockville to a small site at Tysons just outside the Capital Beltway. Mervis said it was a calculated assessment of where the firm's future sales were likely to be heaviest.

"We compared trends and buying habits, available consumers," he said. "Any transaction is far less tortuous in Fairfax. [In Montgomery], they turn over every penny and look at it before spending it. People in Fairfax earn more and don't worry about spending it. They say, `I like it, I want it, I'll buy it.' "

Statistics compiled by Claritas Inc., an Arlington-based consumer marketing-information firm, show that Fairfax's median household income has reached $87,569, the highest in the nation of the more than 3,000 jurisdictions surveyed. It is a lofty perch that Fairfax has held for the past decade, according to census numbers.

In 1990, median household income in Fairfax was $5,195 more than in Montgomery County, long the preeminent symbol of wealth in the Washington area. Now that spread has more than doubled, to $12,671. Median income means that half of the households in each county have more income and half have less.

Fairfax is "in some very fast company," said Ken Hodges, a Claritas demographer, "and by some measures it goes right to the top. It has a large component of affluent households, not many that are real low and not enough to drag down the median."

Consider:

Almost 19 percent of Fairfax households have more than $150,000 in annual income, sixth highest in the country. That's up from 4.5 percent and a 20th place ranking a decade ago. Fairfax trails such places as Fairfield County, Conn.; Somerset County, N.J.; and Westchester County, N.Y. Montgomery County comes in 12th, with 16 percent of its households having $150,000 or more in income.

About one of every eight Fairfax households has accumulated a net worth of more than $500,000, according to Claritas, which places it fourth nationally, behind Nassau County, N.Y., and Morris and Somerset counties in New Jersey. Montgomery is seventh.

With the growing income has come a competitive impatience for the trappings and toys of wealth among many of Fairfax County's best-off residents.

Keith Ryland, a 35-year-old senior director for one of Fairfax County's burgeoning technology firms, wasn't about to lose out on the home site of his dreams -- nearly an acre of land in upscale Great Falls, with lots of trees. The lot and the house planned for it would cost him $720,000.

But Toll Brothers Inc., which owned the lot and would build the home, had decided to sell the secluded property to whoever was first in line at the site at the stroke of 11 a.m. one Saturday this summer.

Ryland camped out on the site till midnight, hired the son of one of his real estate agents to sit with the property through the night and came back himself at 6 a.m.

"I don't like dealing in uncertainties," Ryland said.

Good thing he did, too, because a competing bidder showed up at 5 a.m. But, with a $50,000 down payment, Ryland got his home site.

"It's just amazing the number of young people that can afford these $400,000-and-up prices," said real estate agent Nancy Daniels of the Prudential Carruthers brokerage, who helped Ryland secure his house deal. "People in their thirties with little kids."

To be sure, while Fairfax has become home to many haves, there are thousands who do not have much: A total of 18 percent of the county's 156,000 public school students are eligible under federal guidelines this fall for free or reduced-price meals. To qualify, a family of four must have an annual household income of less than $30,895.

"Any county of this size takes a lot of services to keep it going," said Sharon Kelso, executive director of United Community Ministries, a support group for Fairfax's poor. Many of these service providers cannot afford to live in the county, she said.

Still, it is the upscale side of life that has become as much a symbol of Fairfax as the Washington Monument is of the District.

Stephen S. Fuller, a George Mason University professor, estimated that last year the Fairfax economy -- the value of the goods and services produced there -- totaled more than $45.8 billion. That's larger than the economies of 16 states.

Good incomes have afforded John Gerndt, 39, who works for a defense contractor, and Karen, 35, his wife of three months who works as a hospital wellness coordinator, some pleasurable moments on the golf course. Together, he said, they have a joint income of $120,000, a bit above what Fuller said is the Fairfax average household income of $115,430.

Gerndt said he and his wife play golf once or twice a month at the public Pleasant Valley Golf Club in western Fairfax, a short distance from the $300,000 house they bought in the Virginia Run development.

"You pay $170 to play and throw in a little food and you're looking at a $200 day," he said. "But I don't mind paying for quality. Some of the older courses are not as well kept."

Fairfax car buyers don't mind paying for quality, either.

Saeed Khamenian, a top salesman at the Rosenthal Infiniti dealership at Tysons Corner, thinks he knows why new customers are showing up to look at cars that often cost $30,000 to $40,000.

"These cars are wants, not needs," he said. "I know it's a good economy when I see people looking for their wants rather than their needs."

One software engineer for America Online Inc. who was there at the founding of the computer giant in the 1980s said he didn't quite hit a million dollars in his thirties, but didn't miss by much. He said he collected numerous stock options over the past few years and realized he was a millionaire a month after his 40th birthday.

Now the man, who declined to be identified in order to protect his family's privacy, said he is worth more than $10 million, even after giving away $2 million or so to charities and relatives.

And, while the man recently spent a week-long vacation helping his Methodist church group build a house for a poor family in Appalachia, his wealth has changed some things.

"When I bought a car [a 1996 Dodge Stratus], I didn't take out a four-year loan, I wrote a check," he said.

Now, he and his wife have bought a wooded 13-acre home site near the 12th fairway at the Westfields Golf Club near Clifton for $450,000 and are starting to build a $1 million, 8,000-square-foot house.

"It will be a very smart home," he said. "If a function can be computerized, it will be."