The newer model is better. What kind of info are you looking for? There are a number of GPU comparison sites on the web. Try googling "compare gpu" and go from there.
Until Apple completely fixes the drivers for the 8600m, the 1600 might be a better option. That is in the event the OP wants to play CoD2 or Civ IV, though. Anything else the 8600m should perform better at.
The 8600 is a Direct X 10 compliant video card while the X1600 is Direct X 9. Some features in games are DX10 only, these visual settings will not show up in a DX9 environment. Currently games that take advantage of DX10 also work in DX9, but game makers will switch to DX10 only giving that X1600 a shorter lifespan than the 8600.
The difference between the 128MB and the 256MB are minimal. Barefeats measured it to be something like a 9% difference - really not worth it for the majority of people, especially since the upgrade is $500.