I have the opposite opinion of Jafula on the Nvidia/ATI matter - i've experienced ATI drivers (that have a regular release schedule of monthly) to be more stable.

But the most important aspect that i found - was that ATI was way superior when dealing with LCD TVs and multiple displays. I used to play main on 1080p LCD tv and 4 slaves on 24" monitor.

As for the questions:

1) No, CPU Will handle it fine, you might want to tinker with ProcessAffinityMask (but set this to 255 and forget about it).

2) If you can afford it, get 12 straight away, the more details you turn on your wow clients the more memory they will eat. i personally run on rather low details but end up using ~5-6.5 gb of ram for 5 clients. i hear people have way less mem usage, but i don't know what they do different.

3) you can live fine with 1 video card. get one with lot of memory since you'll be rendering on a lot of screen estate. (1gb - don't know if 2gb would make a difference for multiboxing, never heard of anyone doing a benchmark)

4)For Vista 64 - count 15gb for system, 5gb for other things, 10gb for your documents, downloads, temp storage and 15gb for wow. you'll then have a bit of free space. You should consider having one master wow-folder - and then symlink 5 wows to this master folder (for DATA-folder) - there are several different ways and practices do doing this, i don't know if one is better than the other, but just linking the data folder works nicely for me. (separate wtf config for each client makes it remember which char was last logged on for each wow for example. with a single wow launched 5 times, you'll overwrite the last-logged-on for each time you start).

5) yes you can - you can also buy an external usb harddrive or similar for backups and safe(r) storage. i would put windows on the ssd as well for the increased performance.

6) if you're getting an i7 system you're getting state of the art motherboard, memory and such. your videocard and the components will draw a lot of juice. you should consider putting money in the PSU if you have some spare. a more expensive psu usually means higher quality (personally i run a corsair hx 750w on a c2d quad with 8gb ram and ati 4870 512mb).

*Final Tip: Don't buy a fancy Computer case, get an Antec P182 and forget about it. It'll probably be the most quiet and sturdy case you'll ever owned. When you're in a computer store, look at some cases and tap them on the side. Most aluminum cases will go *plonk, wham, dang, bling* - while the p182 will go *thud, thud, thud* (quieter). You'll thank yourself later on.