mate as graphics crads seem to replace eachother every three months !!!
i would got for a BFG8800GT 512mb card. this is a very good card and can be picked up very cheaply.
mind you any 8800gt graphics card will be better than what you have.
i also suggest getting 3 gig of corsiar ram, this will speed things up a bit.
anything more is wasted if your not running 64bit windows.
so, get some more ram (2 x 2gig corsair ram)
8800gt graphocs card 512mb minimum, 1 gig if your budget will run.
and away you go.
another thing that helps is a decent monitor with a quick refresh rate.
the best being 2ms response time. but the higher you run your resolution, the harder you make your graphics card work.
hope this helps
oh and as mentioned before, get a good beefy power supply, this vastly improves performance too