Forgot to say one Radeon X1900XTX PCI-E x16 512MB card would cost me £10 less than two of the above cards. Which would be the better option? (power supply is 500w)
A 7900GS SLI setup will score close to a 7900GTX/X1900XTX but as you said, will cost a little more and, you'll need a lot more powerfull PSU for SLI then for single...
So i suggest you just get a nice 512MB single or better yet, wait for DirectX 10 cards, From what i've heard nVidia is planning to release it's new "G80" cards this November... AMD64 X2 4400+ @ 2,5GHz Asus A8N32-SLI Deluxe 2GB OCZ GOLD DDR500 Asus EN7900GTX 512MB @ 710/1750 Maxtor 250GB 16MB SATAII WD 160GB 8MB IDE Thermaltake Kandalf STEEL
There's no point running SLi. For the most part, it's a waste of money, but in your case, you're weak CPU will bottleneck the card(s) anyway. Get one decent card & don't go overboard. 512MB?? I doubt it's necessary.