Ok... uhm, here's my opinion.
For your
CPU, get a Core 2 Duo. You can get a 6300 or 6400, or a 6600 if you want more performance. However, if you're going to be overclocking, ditch the 6600 for the cheaper ones (63/64).
As for
gfx, you can get a 7900 GT. Once again, if you are going to overclock, get the cheapest one and bring the clock speeds up again.
As for the motherboard, the Asus P5B series are good. But, if you are going to get a 63/6400 AND overclock it, the Gigabyte DS3 is the board for you.
For hard drives, from personal experience, stay away from Maxtor DiamonMax series. Seagate Barracudas are good. Just look around for a rebate- most hard drives with the same capacities have near-identical pricing.
As for memory, you sohuld consider getting something that's NOT value
RAM. Memory is often over-looked, and it's a BIG thing that affects your computer. Try to find something that is at least DDR2 PC5400/667 MHz. For a gaming PC, it's recommended you get 2 GB these days. Although 1 GB is still OK, you might begin to notice lag later on.
Those are my opinions... thing is, I may have limited you, because I do things in CAD, not pounds

But generally, stick with Core 2 Duos, get a 7900 GT, or a 7900 GTO (like a GTX but much cheaper and slightly lower clocks on the memory).
Get good
RAM, and do not get a cheap
PSU. Something at least 500 W, and around 28 A on the +12 V rail.