[upgrade help] upgrading from a gtx 260 sp216 £300 to spend.

Build as follows: core i5 2500k OC 4.3 Gigabyte z68x-ud4 Ripjaws 4gb 1600 mhz Palit gtx 260 sp216 Xilence 600 w gaming psu with one 8 pin pci-e and one 6 pin 256 gb crucial m4 ssd Lg ips234v 23" 1080p monitor