I am building a gaming computer for my brother for Christmas. I have bought the parts from ebuyer.com and it has cost me 538 pounds (tower only - he already has monitor, keyboard etc). If you feel you are capable of building one yourself then I think it's the best way to do it. Mine has the following spec:
Core i5 760
4GB DDR3 1600MHz
1TB hard drive
Radeon 6850 1GB graphics card.
Should be a very decent computer once built, and I would have enough still for a monitor etc and still be within your budget.