i3 2100/2120, H67 mobo, 4GB of Kingston RAM and the nvidia superclocked 460. It's actually dearer here than other places but I can't link to them sorry.
That 460 can be overclocked even further (850mhz is a safe expected OC) so you are looking at close to 6870 performance for less cash. The 6850 is actually worse than the 5850 which was the 460GTX main rival. I know it's complicated lol
If you want a Z mobo you really need to spend £100ish to get the SLI and Xfire done properly. That £85 gigabyte z-68 is £20 more than the H67 and if your only using an i3 you wont be overclocking it anyway. If you want to future proof the system get a good Z68 or just opt for the cheaper mobo and put the difference in price towards the GPU.
Any problems or questions let us know