I would go ahead and build a new computer. Prebuilts use a lot of proprietary technology, so you can't use the normal parts with it. Typically, the motherboard, case and sometimes PSU are not compatibile with aftermarket parts.
Another problem is that if you get a new board, you'll need a new copy of the OS, even if it's the exact same model. Prebuilt vendors use OEM (Original Equipment Manufacturer) versions of the OS which cannot be transferred from one computer to another. The way this works is that specific copy is tied to the motherboard, so if you replace the board, you are essentially replacing the computer.
I would just go ahead and buy a new set of everything. Take a look at the guidelines for asking advice (link is in my signature). We'll need at least a budget and how you intend to use the machine.
Thanks for advice guys. I'll go with your suggestions. Here are my specs:
APPROXIMATE PURCHASE DATE: ASAP
BUDGET RANGE: 100-200 (I only need a case, PSU, and a Motherboard. If possible please give me a cheap option around £100 and a different option for around £200. See the Parts Not Required section for the components I need to integrate)
SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: Editing film in Adobe Premiere, Post Process in After Effects
PARTS NOT REQUIRED: CPU (Intel Core 2 Quad), Graphics Card (I have an ATI Radeon HD 2600XT 256MB PCI-E), RAM (x2 sticks of 1GB Nanya 2Rx8 PC2 -5300U-555-12-E1-667 and x2 stick of Kingston 512 MB RAM 1Rx8 PC2-5300U 555-12-D1 KWM551-ELG), Hard Drive (I have a Samsung HD321KJ), I also have a keyboard, mouse, DVD Drive, monitor, speakers, OS etc. All I need is a case, psu, and MB compatible with the above components.
PREFERRED WEBSITE(S) FOR PARTS: (ebuyer, overclockers)
COUNTRY OF ORIGIN: (e.g.: UK)
PARTS PREFERENCES: Anything as long as it's compatible with my GPU, CPU, RAM, and HDD
So in total mobo+casing+PSU are already about 150 pound.
The 200 pound is meant altogether or 200 pound for one item only?
If you gonna use 200 pound for all, I think you better save the 50 pound.
Or spend some more to replace the graphic card with : http://www.ebuyer.com/product/191864
That will cost ya about another 128 pound
If you wanna spend more, we can recommend better items, you pay more, you get more, just be sure you need them.