all depends on what your going to be playing tbh .. some games require a very top end pc ..
but yeah 700-800 will see a competent machine that will do most ..
just a quick look at a barebones system ..
http://www.novatech.co.uk/novatech/p...504GN.html#sp8

2 x http://www.novatech.co.uk/novatech/p...h/57701GB.html

would work ok . add an optical drive and operating system and off you go ...