Right, I have a budget of £200 (about $330), and I've decided on some components- basically I just wanted to check that there's nothing much better I could get for my money.
(I already have a case, monitor, PSU, OS, and optical drives sorted).

I'd use my computer mainly for composing music and mixing live stuff, not so much gaming- probably the most complex thing I'd want to run is Project64. I already have a Soundblaster Audigy sound card.

CPU- AMD Athlon X2 7850 AM2 2.8GHz 3MB Black Edition
Mobo- Asus AM2/AM2+ GeForce 8200 DDR2 ATX Audio Lan (onboard graphics are fine, so long as it can run Project64, which should be no problem)
RAM- Corsair Memory 4GBKIT (2X2GB) 800MHZ NON ECC CL5
HDD- Hitachi 3.5" 320GB 16MB SATA HDD

That all adds up to £180... so I have room to make changes here and there. My current system is a 256mb pentium 4 so anything would be an improvement...
Also, if there's some major mess up in the components I've chosen, please let me know- I'm new to this.

Good choices, but for a very few dollars more, you could get a Samsung, Seagate, or Western Digital hard drive.
The Hitachi has had such a long history of problems, that they have to prove they are ready for prime time. Last time I looked, they were still at the bottom for reliability, with only Tri-Gem having more disasters.

OK, thanks for the advice! I've gone for a Western Digital 500GB model.

For you, I recommend something like this:

• Asus M2N68-AM SE2 Socket AM2+ nVIDIA 7025 Chipset Motherboard - Retail - £32.99
• AMD Phenom X3 Triple Core 8650 2.3GHz Socket AM2+ 2MB L3 Cache Processor - Retail - £63.24
• Crucial 4GB (2x2GB) DDR2 667MHz/PC2-5400 Memory Kit - Retail - £35.25
• Western Digital Caviar Blue 640GB HDD - OEM - £47.14

Should be within your budget. All prices are inclusive of VAT, and delivery is free on all items except the motherboard.

Would a 2.3 GHz triple core Phenom be much better than a 2.8 GHz dual core Athlon?
Sorry if this is blindingly obvious...

Anyway, thanks for the tips, Rage!

In most cases yes, but the triple will last you longer.

Well, it is newer technology but if you are not into gaming, the Athlon is a more reliable unit.

OK, thanks for the advice.