The 8200m will probably have a smaller Memory Bus Controller, meaning it probably cant address more than 256Mb of Ram, so having any more is a marketing gimmick.
GDDR2 Ram is considerably Slower than GDDR3
The 8600M GT has ALOT more Stream Processors, higher Clock Speeds, and is an overall better card.
Your MBP is better spec'd in every way. So its either a Bad Port to OS X, or settings difference.
I don't know if its a gimmick....
look here
http://h10025.www1.hp.com/ewfrf/wc/...5875&cc=us&dlc=en&lc=en&jumpid=reg_R1002_USEN
compaq says video ram is capable of up to 895mb
yeah 8600m IS way better and I know my mbp is better spec'd.
the vista rating for my stepdads pc is 3.7 and mines is 4.8
so it's way better. and now i'm gonna max out the ram and put a larger hard drive... haha