Well supposedly one PPE performs about the same as a 1.7GHz P3 (on code that wasn't written to run well on a PPE anyway) which I'm pretty sure is a good ways from a 2GHz G5.
The biggest problem with using Cell in a desktop system is that the cores are not symmetric. The SPEs won't run PowerPC code like the PPE so you can't just take a standard multi-threaded application and compile it for Cell and take advantage of all the hardware. You need to specifically develop it to use the SPEs.
The Xenon CPU in the XBox 360 would make a better desktop CPU for that reason, though the new dual-core G5s @ 2.5GHz probably have better overall performance for desktop use. No idea what the prices are for either if IBM will even sell them to anyone besides Microsoft and Apple respectively.
