Wanting it before xmas, so from a local shop that has been recommended to me, is it possible to make a decent gaming PC for under 600 quid? http://www.web-systems.co.uk/?page=Computers
This should work well. Since there is still some room in the budget, if you don't like how the case I chose looks, you can spend upto £50 on another case. I only picked that case because it's cheap and apparently has good airflow. Just make sure that if you change the case, the new case you pick is an ATX mid tower or ATX full tower.