Intel Corp. today plans to unveil new versions of its popular Pentium microprocessor that are designed to dramatically improve the speed of multimedia software. Some industry leaders hope the new chips will rejuvenate lagging personal computer sales.

The new technology will boost the speed of programs laden with graphics, sound and video by 60 percent on average, according to Intel. Computers with the new chips, called Pentium processors with MMX (multimedia extension) technology, should be on store shelves by the end of the month with price tags from $1,800 to $2,500, industry analysts said.

Retail experts believe some consumers, heeding the advice of many computer magazine columnists, deferred computer purchases during the holiday season in anticipation of both the new technology and lower prices on computers without the multimedia enhancement.

Major computer retailer CompUSA Inc., for instance, reported last week that same-store sales -- sales at stores open at least a year -- rose only 1.5 percent in the fourth quarter, a far smaller increase than previous years.

"This announcement has a lot of people hyped," said Krishna Shankar, a semiconductor analyst at Donaldson, Lufkin & Jenrette Securities Corp. in San Francisco. "Retailers hope this will help them out."

To take advantage of the multimedia speed improvement, software must be rewritten for the MMX chip. Major software firms, including Microsoft Corp. and Adobe Systems Inc., already have released MMX-compatible products and several companies plan to make software announcements today.

Software written for standard Pentium processors and older versions of Intel chips will work on the new MMX chips, but only will have a 10 percent to 20 percent speed boost, Intel said.

The MMX enhancement adds 57 "instructions," the basic commands used by software writers to control the microprocessor, to the approximately 150 instructions that Pentiums already have. "Never before have we made such a dramatic change to what our processors look like to software developers," said Jon Khazam, Intel's MMX product manager.

Four varieties of MMX chips will be released today: at speeds of 166 and 200 megahertz for desktop computers and at speeds of 150 and 166 MHz for laptop machines. Industry analysts expect computers with the new chips to be priced at about $250 more than similar machines without the multimedia enhancement.

Industry analysts say that a number of large computer companies will release machines with the new chips today, including International Business Machines Corp., Compaq Computer Corp., Dell Computer Corp., Gateway 2000 Inc. and Toshiba Corp.

Intel plans to add the MMX technology to all its microprocessors by the end of the year, Khazam said. But computers without the MMX enhancement likely will be for sale through mid-1998 as retailers try to get rid of their inventories, analysts said. CAPTION: A LOOK AT THE MMX What: A new version of the Pentium micro- processor, the computer brain. MMX stands for multimedia extension. Speed: 150 to 200 megahertz, depending on the machine. MMX machines will cost about $250 more than non-MMX Pentium machines. Performance: Improves video and audio about 60 percent; runs from 10 to 20 percent faster than non-MMX Pentiums of the same speed.