Well, 1366x768 isn't as bad as everyone is saying, and it takes at least a HD5770 to power a 1366x768 monitor efficiently and you will be able to play most things maxed out.
A 6870 would be your absolute limit, resolution wise and CPU wise for bottlenecks. You'd max everything out at that resolution and it would provide good upgrade grounds to say 1080p down the track.
Also, Nick, that CPU isn't that great. It's old architecture.