To save some money the radeon HD 6870 preforms fairly close to the 560 ti. Im sure if you overclocked it could be just as good =P
May not want to cheap out your PSU maybe go for the Corsair enthusiast series TX650?
So i suggest this
Mobo- ASRock Z68 extreme3 Gen3
GPU- Radeon HD 6870
RAM- Corsair Vengeance 8GB
PSU- Corsair TX650
HD- Spinpoint F3 1TB ( you didnt list one but just in case)