“The computer in each Mars Exploration Rover runs with a 32-bit Rad 6000 microporcessor, a radiation-hardened version of the PowerPC chip used in some models of Macintosh computers, operating at a speed of 20 million instructions per second. Onboard memory includes 128 megabytes of random access memory, augmented by 256 megabytes of flash memory and smaller amounts of non-volatile memory, which allows the system to retain data even without power,” NASA explains in its January 2004 “Mars Exploration Rover Landings” press kit here(pdf).
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More information on the Mars Rovers here.