Also worth having a look at the new Intel Sandybridge i3 21xx series. A bit dearer in price, so will come close to £500 including a monitor but they are awesome performance (
click me), run very cool and efficiently and are newer technology. Building a system if pretty straight forward, thing generally can only fit one way and assuming nothing is dead then you can be up and running in a matter of hours.
Change the case (& PSU) to something you like the look of but here is a basic Intel Sandybridge system;
Microsoft Windows 7 Home Premium 64 Bit, Operating System, Single, - OEM - £72.08
MSI H67MA-E45 (B3), Intel H67, S 1155, PCI-E 2.0 (x16), DDR3 1066/1333, SATA 6Gb/s RAID, mATX, VGA - £81.20
1TB Samsung HD103SJ Spinpoint F3, SATA 3Gb/s, 7200rpm, 32MB Cache, 8.9ms, NCQ - £37.79
4GB (2x2GB) Corsair XMS3 Classic DDR3 PC3-12800 (1600), Non-ECC Unbuffered, CAS 9-9-9-24, 1.5V - £38.39
Coolermaster Elite 334 Mid Tower Case, Black, with Coolermaster Elite 460w PSU - £50.39
Intel Core i3 2100, 1155, Sdy Bridge, 3.1GHz, 5 GT/s, GPU 850Mhz, 3MB Cache, Core R 31x, 65W, Retail - £87.59
Total - £367.44 inc free delivery.
Leaves you upto £150ish for a 24" screen or save a few bob and get a 22". Getting a 24" screen for me personally was one of my best upgrades. So much space, esp if you have lots of windows and applications open at once. This i3 is significantly quicker and has SATA 6Gp/s and USB 3.0 (hand for future upgrades). You could get a smaller case, that will take mATX boards, if you wanted something a little smaller to save space.