http://www.spacemart.com/reports/Space_Bas...mputations.html

This spaceflying 'puter does 1,000 Giga Operations-per-second and consumes only 80 Watt.
That is amazingly little, but for autonomous navigation for rovers it is still too much.
However, our military friends are working very hard to get the power consumption down, so we will get there!

That is a very impressive number but this is a fairly specialised device (Software defined radio) so you can't really take this and map it across to the operation of a general purpose cpu. Also the term "giga operations" per second would need to be better defined - those ops could be simple single bit "operations" carried out in parallel or they could be complex floating point operations.
2000 Giga single bit operations per second is about equivalent to a PS3\IBM Cell processor running at 2Ghz (128 bit x 8 core x 2Ghz =~ 2000 billion bits\sec). 2000 32 bit floating point ops per second by comparison would be about 250-1000 times "faster" than a general purpose top of the range current desktop CPU.

It's all a long way from the 20Mhz RAD6000's that the two MER's have on board in any case.