Hey! I just upgraded my computer for 5 instancing myself and I was pleasantly surprised with what I got.

I bought an Intel Core 2 Quad Q6600 processor (http://www.newegg.com/Product/Produc...82E16819115018). Make sure you have a Socket 775 motherboard that supports quad core processors.

My 8800GTX is entirely up to the task of keeping 5 WoWs running, even with my main window on maximum details and across two displays. The 8800GT should do you just as fine for half the price (http://www.newegg.com/Product/Produc...82E16814130319).

I went for 8GB of memory after I noticed how dang cheap DDR2 has gotten (http://www.newegg.com/Product/Produc...82E16820231122). Make sure you go with a reliable brand (G. Skill, OCZ, Crucial, Corsair, Ballistix are all brands I've had good experiences with in the past). Remember even if you only go for 4 gigs you're not going to be able to use all of it in a 32 bit operating system.

32 bit applications still run under a 64 bit operating system, Windows uses what's called "Windows on Windows emulation" to ensure backwards compatibility with all 32 bit programs. It's a little sophisticated because it actually has to maintain two separate sets of all your system DLLs and regristries, but it should be completely transparent to the end user. I've had a very good experience with Windows Vista 64 bit so far, and most all of the woes (namely lack of driver support) of the last couple years of 64 bit computing have been resolved.

Two hard drives is better than one because of RAID 0. RAID 0 allows you to split up your disk operations across two drives, theoretically halving the time it takes. Of course, the real world performance gains aren't nearly so perfect, but it is an improvement!

Liquid cooling is only for very serious overclockers who are willing to spend a lot of time and money on getting the last ounce of power from their hardware. There are /extremely/ good and cost effective air cooling solutions for $50-$70 that are still better than most of the entry-level water cooling systems (which would cost hundreds). The Thermalright Ultra 120 (http://www.newegg.com/Product/Produc...82E16835109125) is currently the leading HSF and the Tuniq Tower 120 (http://www.newegg.com/Product/Produc...82E16835154001) is second best. They're both huge though, so you may need to look at smaller heatsinks if you have case room restrictions.

Lastly, I personally recommend two displays instead of one unless your primary display is very large. All of the 8800 cards come with two DVI heads and it's best to use them! Having multiple displays really helps you sort out the mess that you're going to have on your hands soon enough once you get 5 instances up and running.

Good luck!