EDIT: I don't know what I was saying when I said it should come out close to 1600, more like 2200 for the parts I posted lol.
Just get the 6Gb RAM set.
Like baron said the MMO might not support SLI or Crossfire, so it's up to you if you want to go with the 5970 or two gtx 480's. Either way you'll have one **** of a rig
If you go with the 5970, add about a hundred more pounds to the final cost. If you go with two 480's, add about 350 pounds more.
This is a GREAT case:
HAF 932 Coolermaster HAF 932 Case - Black (No PSU) : novatech.co.uk
Great cable management, quiet but awesome airflow, room for water cooling, etc.
And get this cooler:
NH-U12PSE2 Noctua NH-U12P SE2 Dual Fan Quiet CPU Cooler Fitment For AMD AM3, AM2+, AM2, 939 & Intel 1366, 1156, 775 Socket Processors : novatech.co.uk
The previous model in this same series was my first serious cooler, managed to get a 4.63Ghz overclock on my 920 with it, and it'll hold you at 4.2Ghz stable 24/7
Total extra cost will be ~165 pounds
edit: also, for a powersupply you'll def want more than 600W. If you go with the 2 GTX 480's I'd recommend a 1000W power supply, if you go with the 5970 I'd say at least 750W