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 Sega of America on Monday will announce an add-on cartridge for computers that is intended to sharply increase the performance of its Genesis video games while increasing the games' realism.

The machine, which is expected to cost less than $150 and reach the market in the fall, is intended to ease the transition to 32-bit software from 16-bit software.

And the Sega system is one of the clearest examples yet of a significant shift in technology. In the past, the fastest computers were introduced first for military and advanced corporate applications. In recent years, however, the huge volumes of consumer electronics sales have altered the market so that now the fastest systems are quickly finding extremely low-priced consumer applications.

"The tail is beginning to wag the dog," said Jesse Taylor, a Sega software designer.

 Four Times the Memory

The new cartridge, which will work with Genesis cartridge and compact disk models, will have four times the internal memory. It will have two Hitachi microprocessors, each capable of executing 40 million instructions a second. The machine will be able to display 32,000 simultaneous colors and maintain video rates of 60 frames a second, said Jesse Taylor, a designer of the system.

It will come with a graphics capability that falls well into the range of current computer work stations, Mr. Taylor said. The system, named the Sega Genesis Super 32X, will be able to draw 40,000 to 60,000 polygons a second on a television screen. That compares with a high-end Intel 486-run personal computer, which can draw about 2,000 polygons a second, he said.

In the last year, Sega has had the strongest position in the $6 billion video game market in the United States, and the company is moving to maintain its lead over Nintendo in the 16-bit game market and head off challenges from 3DO, Sony and Atari.

Sega overtook Nintendo last year because it was more aggressive in making the transition to 16-bit from 8-bit machines.

Nintendo has said it is counting on a 64-bit system called Project Reality, on which it is working with Silicon Graphics Inc. That system will reach game arcades this fall and will be introduced as a home system sometime next year.

But analysts said Nintendo was vulnerable to Sega's strategy, because it might end up trailing in the introduction of advanced software titles.

"If Nintendo doesn't mimic this, they'll be in trouble," said Denise Caruso, editorial director of Technologies and Media, a San Francisco-based industry newsletter. "This is a good way for Sega to protect their installed base and at the same time move to higher-end software."