more GPU Memory doesn't mean a better card, the DDR3 will out perform the DDR2, the DDR3 is newer technology than the DDR2!!
8600 GT 512MB/GDDR2:
Memory CLock: 1000Mhz
Memory Bus : 128MB
8600 GT 256MB/GDDR3:
Memory Clock: 1400 Mhz
Memory Bus : 128MB
The memory bus widths are the same but look at the Memory Clocks, faster processing, and better performances!! You can't compare GDDR2 to GDDR3, the same way you can't compare GDDR3 to GDDR5...
hope this helps!!
Dane