Ok, for tower only, this is fairly high-end on the core components, with basic but decent supporting components:
Core components (non-overclocking): 483 pounds
i5-3570 as good a gaming CPU as you can get without overclocking an i5, a cheap & basic motherboard that will allow one open expansion slot other than the video card, a high-end video card, the 7950, and some RAM.
Supporting components: 222 pounds
NZXT 210 Case, a basic but decent case. White with black trim, also available in all black. Be Quiet S6 450w power supply. Normally I'd recommend the XFX Core 450w as a great deal, but it's out of stock at the moment. Samsung 840 SSD (optional) put your OS & the programs you want to load zoom-zoom fast here. 1TB HDD for general storage. DVD-burner (optional).
Optional Sound Card (some people who play shooters like these for pickup on footsteps - not all that useful without quality headphones or speakers; will use up last expansion slot on the motherboard, but I dunno what you'd put there anyway): 48 pounds
Total w/o Sound card: 705 pounds
windows & anything not-tower related is not included
The main way to spend more is to move to an overclocking configuration (another 50-70 pounds), increase storage size, or go for a quiet-computing configuation with a better video card cooler, higher end case, and that sort of thing. But none of that is going to dramatically increase your general gaming experience.Edited by MisterFred - 2/20/13 at 9:31pm