Currently I run 5 characters on two computers. My main runs on one system and the four alts on the other one. The one the alts run on is a Q6600 processor at 2.4GHz (not overclocked), with 2GB of RAM. I have two accounts running on one hard drive and two accounts running on the other. The reason more hard drives can be better is that they are sending data on separate channels. Instead of having four programs sending data back and forth over one channel, I have them sending it over two. I don't know if four WoWs is enough to saturate a single SATA channel, though. Maybe one drive would be enough.

I am running four copies tiled on a 1600 x 1200 screen (800 x 600 each) with most settings minimized on a GeForce 7800GTX. I run at 1920 x 1200 with most settings maximized on a GeForce 8800GTS on my main system, so I would assume that an 8800GTS should run four WoWs in smaller resolution just fine. The 7800GTX runs those four 800 x 600 windows at around 20-40 FPS, at the least it is smooth enough that there is no noticeable stutter and I do not have any real lag issues (I use keyclone to send the data to the four WoW from the main system, and there is a consistent ~0.5 second delay most of the time, but nothing worse).

I would go with the Q6600 and 2GB or 4GB of memory and two hard drives and a GeForce 8800GTS. I would only go with a 64-bit system and 8GB if I had a specific use for it that I knew would benefit from that. Four WoW under a Q6600 with 2GB works fine for me.