If you don't want to spend too much, I'd probably go with an AMD Athalon quad core at the moment with DDR3 (so you can upgrade the CPU to one of the better phenoms a couple of years down the line).
The core i5's and i7's are a little bit too expensive, and although there are some good budget socket 775 CPU's around, the socket is in the process of being killed off...
AMD Athalon X4 620. Can't really beat a quad core CPU for £75.
You might as well get a DDR3 motherboard, and DDR3 ram. I generally prefer Asus motherboards over other makes - seem to be better quality. So, a perfectly decent one:
Asus-M4A785TD-M-EVO
Or if you want crossfire at some point:
Asus M4A78T-E
For memory, I'd recommend getting the fastest DDR3 you can afford (does help when overclocking, and you can set the memory speed in the bios if not). 4Gb should be ok for now, and you can always add more later...
4gb DDR3, PC3-12800, 1600Mhz
If you are going to overclock it, get a decent heatsink and fan such as
this. Keeps it quiet as well
(see
http://pcityourself.com/forum/showthread.php?tid=11 )
A fan controller is a very cheap bit of kit, that keeps you sane when you're not gaming
Might as well get a
blu-ray drive as well (make sure you get one that can burn DVD's as well! There are a few reader-only drives out there)
If you think you might want SLI/crossfire at a later date, make sure you get a power supply that can handle 2 graphics cards. I'd aways recommend getting the highest efficiency PSU you can afford (saves money in the long run, and helps the planet too!)
As for graphics cards, might as well get an ATI one imho, since both the motherboards I listed above support hybrid crossfire (will save on eletrcitiy when you're not gaming). I normally just go for the latest, but somewhere near the bottom of the range. (easy to upgrade again in future)
1GB Asus HD5770
After that you just need a case, a copy of windows 7, and a hard drive or two....
