Can be done cheaply, but you've gota question if spending 4-500 quid on one that does the job now is worth it when new games demanding greater physics and power will be arriving when/if the new PS3 is launched, UT2007 for example next May will require alot of gaming umph.
I'd recommend getting the best sli mobo you can and compromise on a less expensive PCI-e graphics card and processor. Doing that you will get a decent machine for now but will enable you to spend a bit more in the future. I play everything turned upto High on me upgraded Dell D4600. Specs are;
P4 3.2 Prescot
2.5Gb PC3200 DDR2
128mb 6600GT (AGP) OC'd
300 watt PSU
Audidgy 4 sound card
I get roughly 6500 marks in 3DMark 05 and a very high figure in Aquamark compared to alot of people running a similar spec. I run CS Source, HL2 DM, Quake 4, UT2004, FEAR all on full detail with everything turned on using a 1024x768 resolution and get 40-50 FPS in all my games, during large battles or games with alot going on it sometimes drops as low as 30 but its usually pretty consistant.
Unless you wana go Roy's route and do the big upgrade now, doing it on a budget and getting a good gaming rig can be done cheaply.