Nehalem will probably start with a very expensive Extreme Edition CPU, for £500+. AFAIK it will also require DDR3, again very expensive. I don't think you'll be happy if you wait for Nehalem just to discover you can't afford it.
If you decide to build these days, you could probably get something like Q9450, HD 4870, GA-X48-DS4, 750TX
GA-X48-DS4, 139 GBP
http://www.scan.co.uk/Product.aspx?WebProductId=819510
Q9450 retail, 204 GBP. I'm assuming no overclocking, no aftermarket cooler. I wouldn't overclock a work machine.
http://www.scan.co.uk/Product.aspx?WebProductId=743028
Corsair 750TX, 76 GBP
http://www.scan.co.uk/Product.aspx?WebProductId=684732
RAM, 4GB, Corsair DDr2-800 4-4-4-12, 70 GBP
http://www.scan.co.uk/Product.aspx?WebProductId=766626
Sapphire HD 4870, two of them , 185 GBP *2
http://www.scan.co.uk/Product.aspx?WebProductId=836590
WD6400AAKS Western Digital 640GB, two of them, 57 GBP * 2
http://www.scan.co.uk/Product.aspx?WebProductId=798906
CoolerMaster RC-1100 Cosmos, 164 GBP
http://www.scan.co.uk/Product.aspx?WebProductId=771271
Maybe get a cheaper case. I wanted to recommend RC-690,
which should be a lot cheaper, but scan doesn't have it. Shop around, maybe www.ebuyer.com or www.overclockers.co.uk.
The case is a personal taste thing anyway. Just make sure it's big enough and has lots of fans.
Total 1137 GBP. Add Windows and shipping.
If you want to overclock, add these Xigmatek products for 26 GBP
http://www.scan.co.uk/Product.aspx?WebProductId=824662
http://www.scan.co.uk/Product.aspx?WebProductId=700899
and get the Q9450 OEM to save 5 GBP.
http://www.scan.co.uk/Product.aspx?WebProductId=795731
Edit: forgot the DVD.
SH-S203P, 17 GBP
http://www.scan.co.uk/Product.aspx?WebProductId=815123
Edit: I'm assuming you didn't include monitor/speakers/mouse/keyboard in the budget. If you did, get only one video card and only one hard disk for now, and a cheaper case.