It's really about how much you want to spend. The firm in the link below is trusted by many here, and I can vouch for them also.
You could of course build your own top spec pc, but a few golden rules to be aware of whichever way you go:
1) Go for a Quad core CPU if you can afford it
2) At Least 2gb RAM (better with 4gb)
3) A top notch Graphics card in order to play your games at a decent frame rate and resolution.
Some other links here:
Graphics cards reviews above.
Spec your own in the link above.
This will give you an idea of the kind of money you need to spend. I build my own so can't vouch for the retail built range from any company, but many here will have recommendations.