On $650 an i5-2500k and a GTX 560Ti I don't think is really possible.
Your looking at a $200 card and a $230 CPU.
For similar performance an i3-2100 build is going to be your best option. http://www.tomshardware.co.uk/gaming-fx-pentium-apu-benchmark,review-32368-10.html
i3-2100 - $125
Z68 Motherboard ASRock Ex3 Gen3 - $110
Gigabyte GTX 560Ti - $200
500w PSU - $50
Case - $50
HDD 500GB? - $80
Optical Drive - $20
4GB RAM - $30
Prices are a rough idea and not 100% accurate. That build comes to about $650 though.