Sounds like a good choice to me, you'll be very happy with it.
Memory size matters the least, core/mem clocks equal importance but not the most important factor, and memory type is most important (i.e. 128-bit or 256-bit, DDR or GDDR3). Of course Pixel Pipelines is most important of all.
As an example, a 6600GT and a 6800 comparison. These cards are around a similar price.
A 6600GT has a core clock of 500MHz and memory clocked at 1GHz, both clocks are higher than a 6800 which is clocked at just 325/700. However, the 6800 is the faster card overall, since it's memory is 256-bit (as opposed to the 6600GT's 128-bit only interface) and it has four more pixel piplines and two more vertex units.
Memory size only really counts in high resolutions, especially 1600x1200 where 256MB is useful. 128MB is good enough up to 1280x1024. Having more memory will not boost your frame rate at a lower resolution at all.
EDIT: 6600GT is a bit on the expensive side, I was lookin at either Radeon 9600 or GeForce 5700. What's the comparison between 6600 and 5700 for GeForce?
6600 is about 2 to 3 times faster than a 5700.