Hi djayschneider and welcome to the Tom's hardware forum.
1- U can buy the GTX460 and install it without problem in the PCI-E x16 slot of ur mobo.
2- For $500 u can go with this:
GPU: ATI 5770
CPU: Athlon II X3 445
RAM: G.Skill Ripjaws 1600
Mobo: Gigabyte 780A-UD3
Total: $492.18 including shipping but not MIR.