Right seeing as your gonna be looking about for parts...
Heres the stuff you will NEED for your computer to work
IMPORTANT!Motherboard
CPU/Processor
RAM/Memory
Graphics Card
Hard Drive
DVD Drive
Power Supply
Case
Optional extrasSecond Graphics Card (make sure theyr the same and your motherboard supports it nVidia=SLI ATI=Crossfire)
PCI/PCIe Sound Card (onboard is fine but seperate X-Fi is better)
PCI WIFI, network or 56k modem (can also have usb for wifi but internal is easier to manage and usually means better signal, Wireless N is the latest standard)
Extra hard drives for extra storage (RAID requires 2 or more of the same drives)
Extra DVD drives so you can load more than one disc at a time
I/O panel for a drive bay (can be a card reader, extra usb, headset sockets, firewire ports, floppy drive ect..)
Fan Controller (make your fans run slower and thus quieter)
Aftermarket CPU cooler (can range from a fanless for silent to super loud but super cool, or even a water loop.. the stock one will do fine if your not overclocking and you dont live in a dessert)
Remote control receiver (EG the iMon 3.5" bay receiver can turn on your pc, control music/video and access the media center with a remote control)
Things to rememberMobo stuffThe intel chipsets (P45, x38, x48 ect) support ATI Crossfire and the nVidia chipsets (680, 690, 750, 780, 790 ect) support nVidia SLI multi card setups
You may not want 2 graphics cards now but in the future it will be a nice upgrade option for future games so you may want to make sure you have 2 PCIe x16 slots on your motherboard.
the X58 chipset is for the new Intel Corei7 and supports both CrossFire and SLI but is very expensive and out of your price range
some trusted brands include Asus, DFI, Gigabyte, XFX, MSI, Foxconn ect
Also make sure it has enough internal and external USB/FireWire ports for your needs
CPU stuffYou want a dual (or more) core processor for your computer to be competative with games of the future.
anything about 2.5Ghz should do the job with some exceptions.
bigger numbers dont always mean better. at stock speeds, an amd x2 cpu will often beat an intel core2duo but intels are prefered due to the massive overclocks that can be had without much trouble
but make sure your motherboard has the right socket for the cpu.. (LGA775 for Core2Duo and AM2 for AMDX2)
RAM StuffMake sure you get the right speed for your motherboard (check the specs)
PC2-6400=800MhzDDR2, PC2-5300=667Mhz DDR2 ect
DDR3 is the new standard but is very expensive when compared to DDR2
Some good trusted brands include OCZ, G.Skill, Geil, Corsair, Crucial ect
Graphics cardsMake sure its DX10, has at LEAST 512mb of vram, PCIe 2.0x16 and fits in your case
nVidia and ATI both offer some awsome performance for the prices atm.. but the 4850 1gb is considered the best bang per buck card by most people on the internet and magazines.
I could go on forever explaining the ranges.. but theyre very confusing..
Sound cardsX-Fi is the best way to go for games... they will improve performance and sound better than onboard sound.
Asus make some great sound cards but theyr better for watching movies and music than playing games.
Game developers optimise games for X-Fi so if you have a creative card in your pc, the game will sound how it was made to sound.
Hard DrivesMake sure its at least 7200RPM, Sata II and has enough space for you!
Installing a RAID config is easier than ever on the latest motherboards. Vista doesnt even need you to load drivers for onboard RAID. If you do choose a RAID config, make sure you buy 2 identical hard drives or you will get lots of errors and your array will fail instantly (or almost instantly)
DVD DrivesMake sure its SATA II, Dual Layer, 20x and can write all media types (apart from blu-ray unless you have £150 to blow)
PSU/Power SupplyUse
This web tool to see how much power you will need..
If your thinking of going for a multi video card setup in the future, dont forget to factor that in, if you dont, you may need to upgrade in the future!
IMO you should never scrimp on the power supply as if its not good enough or it fails, its an expensive mistake that could have been easily avoided!
Buying a power supply that is rated higher than your components can draw will increase its efficiency meaning your electricity bill will be lower

Make sure its got the 80+ Certificate and can handle 2 of your cards for the future, meaning you wont need to buy another if you add another card
CaseThis is mostly down to personal taste.
Make sure it can house your graphics card, is ATX compatable, has enough bays for your hard drives/DVD drives/IO panels and has good airflow to keep your system cool and youll be fine!
other than that, make sure you like the look of it and dont mind it being on show

bloody hell that took ages!