hmm, where to start! Well I would say steer away from pre-made machines from places like Dell - you'll spend a hell of a lot more than the parts are worth. The best and cheapest way to buy a new PC (especially a gaming one) is to get each bit seperately and get a friend in-the-know to build it for you. Or if you don't know anybody that can do that, take the parts to a shop and they'll do it for you.
On the subject of components though, if you want a decent gaming PC you'll obviously want a quality GPU and RAM, but CPU is super important aswell. You could spend say £450 on a top graphics card + 2GB RAM, but if the CPU is not at their level then it will severely limit the performance. So make sure to balance the majority of your budget over those 3 most important things. But for a complete new build and new processor you'll probably want a new motherboard too.
Just as a guide, I built this for a friend the other week (gaming machine):
Athlon 64 3800+ processor (£170)
ASRock S939 motherboard (£50)
ATI Radeon X850 XT (£180)
Corsair 2GB DDR RAM (£130)
Prices are a rough guide but that's <£600 for a pretty decent gaming machine, which you may spend over a grand on at somewhere like Dell.
Worth thinking about, because with the extra money you save you can get a very nice monitor and top quality case
but was told DELL was a good name.
