Whilst upgrading the graphics card may improve things in the short term, your cpu is going to cause a bottleneck. Game manufacturers will give the minimum specifications required to run the game (cpu and graphic card etc). You need to stay above minimum for reasonable results.
In my opinion, if you wish to play the latest games, then it may be better to purchase a 'motherboard bundle'. This consists of a new motherboard, cpu and ram. Using 1gb ram is common these days with XP.
The 6600GT is a good card, the 6800 is better but it all depends on how much you are prepared to spend.