here is what i have done the last 2 times
1) buy computer buyer mag - it has decent reviews and loads of adverts for sellers, you may find something you like
http://www.computerbuyer.co.uk/
e.g.
http://www.computerbuyer.co.uk/reviews/242691/pc-specialist-apollo-q8200-gtx.html
this will give you a good idea of wha tyou get for should expect for your money
2) make a wish list of components - what motherboard, processor, graphics, memory , monitor I want
3) revise wish list to make it realistic with budget