DDR3 Rams have higher frequency but u need a processor to take advantage of it
a Core i7 with DDR3 memory vs a Core2Quad Q9650 with DDR2 Memory
although both processors have similar performance but Corei7 will have advantage bcoz of faster DDR3 Rams and the ability to utilize it
as for VGA Cards DDR3 iz the way to go
the memory of VGA Card iz only for the card itself not for the entire system and DDR2 ones just plain suck
i wuld suggest u get GTS250 for a 19inch LCD 1440X900
or GTX260 for 22inch LCD 1920X1080
for GTA4 ur E5200 needs to be overclocked
all other games will run smoothly
one more thing get E6300 2.8Ghz instead of E5200 though