Two years ago I put together the below system for a bit a gaming, and mainly thanks to Steam I've used it much more than I thought.
Motherboard GIGABYTE GA-Z68MA-D2H-B3
CPU INTEL CORE I5 2500K
RAM Kingston HyperX Blu KHX1333C9D3B1K2/8G, 2x4GB
Hard drive OCZ VERTEX 3 120GB
GPU EVGA GeForce GTX 560
PSU SliverStone Strider Essential 500W
Case Antec Three Hundred Mid Tower Case, Black
Monitor LG IPS236V 23" 5MS IPS
Overall I've been happy with it but it is starting to chug on some of the newer titles (Farcry 3, BF4 etc). All my gaming will be done at 1080P on a single monitor (either the 23" above but more often the 55" TV in the lounge), I do like to crank AA and AF right up.
Is there a GPU that would slot straight into this system without too much headache? Budget is around $500ish, would like to stay with Nvidia if possible. PSU also might be an issue if get a powerful GPU I guess, again would prefer not having to upgrade that at the moment.