Yes, if you so SLI then you will either get higher frame rates, or it lets you turn on more eyecandy while keeping the framerate high. Ignoring the rest of the computer this is the order of graphics processing power:
7900GT - £220
X1800XT - £220
X1900XT - £330
7900GTX / X1900XTX (equal) £370
7900GT SLI - £440
7900GTX SLI - £760
X1900XT crossfire - £ 700
Any of these counts as high end gaming, but after the GT SLI option it starts getting silly. The 7900GT option is one of the rare cases where buying SLI makes sense. Check out this review of the 7900 core
http://www.anandtech.com/video/showdoc.aspx?i=2717&p=6 to see what sort of benifits you get from each system. Remember that the graphics settings they are using there are very high.
Assuming you're going for either a X1900XTX or a 7900GTX or the 7900GT SLI for the meantime and as such you've got 1K to spend on the rest of the system.
CPU- I'd go for the 4200X2 for the CPU, costs £240. While dual core doesn't make much difference at all at the moment for gaming it's nice to have and the 4000 is only £20 less.
RAM -Rather irrelevant, PC 3200 CAS 2 RAM from a good manufacturer, crucial, kingston, OCZ, Gskillz etc. The performance differnece of low latency RAM over the ultra cheap stuff is small, only a few %, but you're buying high end so what do you expect
Should cost around £150.
Motherboard - If you're not overclocking any Asus/Abit/Other good name motherboard should be alright. If you're not buying SLI then don't get a SLI board, as an upgrade path it's a bad plan. £60-£150
Graphics < £500
Case - Antec P180 (as an example), £85, it's designed for high end computers either with excellent cooling, or with ok cooling and very low noise. If you don't like the look of it or find another one you like more then there's no reason why not.
PSU - Enermax liberty £75 Modular, quiet, a very nice PSU by all accounts, enough to run SLI on, and a lot more. Other options are around for around the same price. If you find a cheaper one you like the look of stick up a link.
Hard drives - Depends,
If you want the fastest of fast then the raptor 150 is it, no questions asked. 150GB for £200
If you want a better value hard drive that is very fast, and yet has a decent amount of storage then the WD 4000KD is hard to beat 400GB for £144
If you're not that fussed about how long your map loads are and don't need that much room then a 250GB drive is the best bang for buck, Seagate have a 5 year warranty, WD and hitachi are both very fast, Samsung is quiet and maxtor is very cheap. £70-£200
Sound card - Your old creative one isn't a good option in all likely hood, unless it's an audigy 2 or better then i wouldn't bother with it over onboard. If you do want a good sound card then the XFI range is pretty much it, or if you want a cheaper option then the audigy 2ZS is great value for money, i got the platinum external one with a break out box for less than the cost of an XFI. £0-£80
£680 - £980 plus graphics cards.
There, that's as good as it gets. Assuming you've got a 1600x1200 monitor then i don't think that the £500 limit to the graphics cards will be a handicap.
