[Merkwürdigeliebe]

Building a computer's pretty simple, to build a fully functional tower you'll need:
-a case
-motherboard
-memory
-graphics card
-processor
-power supply (some cases come with a psu, but they are garbage)
-hard drive
-optical drive (dvd-rw)
-operating system (windows, linux, etc)


I'm not too familiar with British pricing, but Intel/Nvidia is definitely the way to go (for now, there are some exceptions)

quad-cores are dirt cheap here North America, see if you can fit a Q6600 into your budget. If not, then go for a dual-core e6750, unless you see an E8400 for the same price or cheaper (again, I don't know how well balanced British pricing is over there)

I highly recommend a Gigabyte GA-P35-DS3L or Abit IP35 for a good socket 775 motherboard for cheap, it'll give you solid performance, even if you decide to overclock

case, is really your choice, any mid-tower will do, I recommend the Coolermaster CM690

memory, 2GB of DDR2-6400, definitely get a pair of Crucial Ballistix, but if it busts the budget, than any pair from OCZ, Patriot, G.Skill, etc should be reliable

hard drive, look for whatever fits your budget and whatever amount of space you'll need, Seagate and Western Digital are pretty much your only reliable choices, I guess you can go with Hitachi or whatever other hard drives are available if the price is good, generally anything out there will do the trick, but the above two I've mentioned are the most reliable, just look to get a SATA2 drive, 7200.11 rpm, 16mb cache

optical drive, not entirely important, just look at the reviews and make sure it ain't noisy or anything, maybe lightsribe or dual-layer is important to you..

graphics card, I'd recommend an 8800GT, but if you want to cut down the price a little, a 9600GT might suit you, or see if an HD3850 will even be cheaper

and as for power supply, if one comes with a case, it's garbage for sure, the Antec Neo HE series is good (the 550 and 500/520 are cheap) and the Corsair 520HX is a sure bet, and its non-modular variety, the 550VX, there's some others, I reccomend just investing in the 520HX and you'll be set for at least 3 hardware generations to come (as long as you stick to the mid-range of parts)

as for operating system, Linux isn't mainstream, Mac OSs need to be hacked on to PCs, so Windows is your best bet, I say stick to XP for now because it'll be cheaper and Vista doesn't offer anything that much better, DX10 hasn't shown any major benefits as of yet, and XP just runs a lot smoother and it's tons more reliable

I think that about sums up all the different parts and the choices I'd recommend