Probably something around 1600 x 1200
Well let me ask you this, what resolution does your monitor support?
For 1600x1200 a GTX460 will do fine you might want to overclock your cpu a little to at least get 2.8Ghz out of it, if not upgrade it to something at least 3.0ghz.
Dougx1317 recommendation is ok too.
Remember to do your own research to come up with the conclusion.