i7 is (supposedly) the future of computers. with hyper threading it allows 1 physical core to act as 2 logical cores. however 2 logical cores can still only handle the same amount of load as 1 physical, this can provide for much faster performances in software that takes advantage of hyper threading. currently the i7 is ahead of its time by a couple years and not very many programs take advantage of hyper threading. one main thing about the i7 that makes it better then most current CPU's is that it take triple channel memory, verses older CPU's that only processes dual channel memory (3 is %50 more then 2 as you know, or should anyway =P). this provides much better performance then other processors, especially for WoW. WoW is a RAM hog as i'm sure you know and the faster the RAM can processes the better performance you can get out of the game.

i7 MoBo's only use DDR3 memory, meaning you will be spending probably ~$150 minimum for 6G of RAM. the cheapest i7 MoBo is $200?, not sure exactly how much it is but a decent one is probably more along the lines of $250. now the cheapest i7 processor is $300, that means that with just the CPU, RAM, and MoBo you are already at ~$650 - $700; this means tat you only have (acording to your budget) ~$300 to spend on PSU, GPU, case, HDD's, a CD/DVD driver and a OS (hopefully you know what all these parts are). these will take you well over your budget, here is and example:
CPU: $300
MoBo: $225
RAM: $150 (6G)
Case: $60 (Antec 300)
PSU: $75 (650w - 700w)
GPU: $230 (Radeon HD 4870)
HDD's: $100 (2x 250G 7200 rpm HDD's)
CD/DVD Driver: $20
OS: $100 (Vista 64-bit)

Total: ~$1,260 give or take about $30 - $50 for better or wors stuff

so basicly if you were looking to spend maybe $300 more you could get a decent i7 build for multi-boxing.

Hope this helps some. ^_^

~YYF