Like other have said.. You'll want a computer with DirectX 10 support, so whatever card you buy you'll want to make sure on that. I always reccomend Nvidia (Geforce) cards as they as usually well assembled by a reputable company. Personally I stay away from PNY.. Go Asus, BFG, MSI, etc..
Since your from the UK (I guess? pounds and all) you'll have to find a place over there that sells things at good prices..
The MSI NX8600GT won an award from HardOCP - and that says a lot about the product. It's $114 USD at newegg. Which is pretty cheap.. It could be a steal. However it is a 128 bit card.. We live in the age of 256-bit cards now. You could find a better model probably
With Intel, go Core 2 Duo.. You don't need quads, there is NO realistic use for them for gaming and casual activities. Unless you like to run shitloads of SETI processing or something.
You also want at LEAST 2GB of RAM. Trust me on this.
For sound, I would get a Sound Blaster X-Fi Extreme Gamer card minimum.. Don't get any model below this as they do not have some of the features the upper end has. Logitech is the only name you will ever need to know when you shop for PC speakers.. Their systems are excellent. They have a great 5.1 System (front, rear, sub) that is usually decently priced in online stores.
Hard drives are cheap.. Find a good price on a 500GB Seagate. I got mine for $99 USD at newegg
Flat Panel Monitors are the way to go.. I prefer traditional versus widescreen but either is up to your preference.. I have a Samsung Syncmaster 931B model. It is great and I love it.. although I did pay retail at best buy incase I ended up with dead pixels and needed to exchange it. I got it last year for my birthday and have had no issues except for recently. I had a stuck pixel (NOT DEAD), that was stuck on the color red.. A soft cloth with warm water did the trick with a gentle massage.
http://www.samsung.com/ca/products/monitor/lcd_digital/ls19mebsbdxaa.asp
I hope this info helps you out.