Thanks for ur suggestion...
:idea: but won't 512 mb be enough ??
(since it is 800 mhz and not 667 mhz)
The speed matters little, especially since unless you are overclocking, you aren't using that speed anyway. Your system is likely showing PC2 4200 which means your running it a DDR2 533 not DDR2 800, and even if it does show PC2 6400, your using a divider which isn't exactly optimal.
You need at least 1Gig if your going to game, hell, if you have more than a couple of apps open you'll want a Gig.
If you went with DDR2 800, to get your moneys worth, you have to overclock. Otherwise you should have saved yourself the money and gotten generic PC2 4200 / DDR2 533 (these mean the same thing one is theoretical bandwidth the other is effective bus speed).
As for the graphics card, the X1950XT or Pro will serve you well. Unless you want to go for a 8800GTS (which is a lot more expensive), I would stay away from equivalently priced nVidia cards, ATI's DX9 cards rule right now.