The 7850 when overclocked would be hugely faster than the 7660D or 6670 (more like 3 times its speed), but I would strongly sugguest you get the 2GB version instead of the 1GB version (as more and more games uses more than 1GB at 1920 res)...but it would be another £20 extra (I know...it's always a little bit more here, a little more there...but I would go as fast as saying 2GB vram is a need for graphic card thesedays).
Also, 8GB of ram instead of 4GB if you can. I know it's gonna be overbudget...but I'd rather spend the little extra and do it right, than to regret it later.
Also with the Z77 chipset motherboard, you'd have the option to upgrade to i5/i7 and overclock in the future. That Gigabyte board will have some limitation for overclock as the voltage would be a bit restricted (might only hit around 4.2-4.4GHz, but the other mini-itx Z77 board that can overvolt and overclocked further (4.5-4.7GHz) would be like another £30-£40, and I doubt you are planning on going for the higher overclock for a mini-itx case anyway considering the temp and cooling.