With two new families of processors introduced by rival chipmakers Advanced Micro Devices Inc. and Intel Corp. last week, computer shoppers have yet another measurement to decipher -- now they'll have to count cores as well as clock cycles.

Both AMD's Athlon 64 X2 and Intel's Pentium D use "dual-core" designs that put two processors on a single piece of silicon, divvying up the work between the pair. These models are available in top-of-the-line desktop computers from such vendors as Dell Inc., Hewlett-Packard Co. and Alienware Corp.

Dual-core technology allows chip manufacturers to get more oomph out of the same space, without incurring the same penalties as conventional designs.

Single-core chips consume vast amounts of power and radiate copious amounts of heat, and going much faster will send those two factors off the charts, said Jeff Austin, marketing manager for desktop products at Intel. But snapping two slightly slower chips together can keep performance climbing.

Contrary to what the dual-core name might suggest, however, these new chips won't deliver twice the speed of their predecessors. The two 3.2-gigahertz cores in a new Intel Pentium D won't necessarily yield 6.4 GHz worth of performance.

Most software today isn't built to take advantage of processor setups that split the job among multiple cores. An application running on a Pentium D that hasn't been revised properly will, in effect, only see a single 3.2 GHz processor.

To take advantage of the other half of the processor, software developers will need to redesign their applications to perform more tasks simultaneously, instead of working sequentially through a list of tasks.

Roxio, maker of popular lines of software for editing and burning CDs and DVDs, is one of the relatively few software companies that has already begun this redesign work.

Uri Kozai, a director of engineering at Roxio, said that crafting software for dual-core processors is "way more complex" than shipping programs for last year's processors. "You have to think in various axes at a time," he said.

But the result, said Kozai and other dual-core advocates, will be a better computing experience, better than what customers would get just by upping a single-core chip's speed or adding memory.

For now, the only users likely to see a big benefit from dual-core processors are those who edit massive video or photo files or those who run multiple resource-heavy applications.

But AMD and Intel say they plan to roll out their new chips aggressively. By the end of 2006, Intel plans to have more than 70 percent of its desktop and laptop processors dual-core, with that figure rising to 90 percent by 2007.

In the meantime, the advent of this technology is already making comparison-shopping between Intel and AMD processor lines even more complicated.

With dual-core processing, clock speeds matter even less than before as a way to gauge performance. The two companies are assigning numbers to their new chips rather than ranking them by GHz. Intel's Pentium D processors are numbered 820, 830 and 840, for example; AMD, which already uses a numbering scheme on its single-core processors, will extend that through its dual-core line. But you won't be able to compare the number of an AMD dual-core chip with that of an AMD single-core processor -- much less make the same comparison between AMD and Intel hardware.

People who simply must know which company makes the better processor can run benchmark tests that report how well a processor performs a set list of tasks. But different tests tend to turn up different results -- some early reports have AMD's new line faster at games, while Intel's are apparently faster at burning DVDs.

Some users won't care -- just as they've ignored earlier advances in clock speeds.

"The only really reliable way of doing this is to look at a group of benchmarks and try to assess the relative importance of various ones," Roger L. Kay, an analyst at research firm IDC. But "no normal person is going to have that level of interest or knowledge."