I can now give some good advice regarding multi-boxing on one computer. 5 accounts, 5 unique directories, using keyclone 1.8k.

My first computer setup:
ASUS PB5 motherboard
nVidia 7300 GT 256Mb
2.0GHz Core 2 Duo
2Gb RAM
1 SATA Hard drive (3Gb/s transfer rate)
2x NEC 1770VX LCD monitors
350W power
Windows Vista Business (its my work computer... yes... I frag at work... self-employment rocks)

Results: Game ran, but man was it slow. I could get it running but I could not do that much outside of the starting zone because of the number of players being rendered. All graphics were turned down to minimum, with only my main's terrain range being set to middle. I wasn't getting anything better than 15 fps in each one.

My second setup:
Moved my 4 alts over to my server which has a raid setup. I think any improvements I saw were mostly psycho-sematic (i.e. in my head). SATA's transfer rate is so high its near impossible for 5 versions of this game running at the same time to trip it up. This isn't your dad's 600Mb IDE drive anymore...

My third setup (upgrades):
Quad-core Q6600, 2.4GHz
4Gb RAM
500W power

Keyclone affinity that worked best:
CPU 0 (nothing... leave for system)
CPU 1 Alt, Alt
CPU 2 Main
CPU 3 Alt, Alt

Results: Game ran considerably better and had a better time keeping the frame rates up. However, I still didn't get anything better about 24fps, and if there were a lot of people around or if I hearthed or went into a town it was near unbearable. Hard drive was doing a lot of work and since I knew the harddrive could handle the games them selves my only conclusion was that the graphics card was pushing a lot of work to the harddrive.

My Final setup (upgrade):
Gigabyte nVidia GTX280 1Gb RAM

Results: HO.. LY... CRAP! Keeping the graphics turned down on my alts, and turning up some of mains graphics I am getting 50+fps on my main and about 45+ on my alts. I can actually fly on all 5 mounts at the same time (from fly points) and it will still render all 5 windows! Insane! I don't have to wait 2-3 minutes after all my toons land to get moving again... maybe 45 seconds. I could run through Undercity with no problem, engage mobs without fear of a hiccup... proof that you can run 5 instances on 1 computer effectively. According to the Windows Experience Index calculator, the bottleneck is my RAM now at a 5.6 score, followed by my CPU at 5.7, and the graphics card and harddrive are 5.9. So unless they increase the speed of RAM significantly or Blizzard starts writing their code in 64-bit this is as optimum as its going to get.

UPDATE: Be careful getting on the boat or zeplin... i just continuous lagged between the two zones until i got booted.

Oh, and to crush a myth... World of Warcraft is a 32-bit game, so running it in a 64-bit operating system will not improve things. Even if you run 8Gb of RAM, the 32-bit emulator only seems about 3.2Gb of it. I got this from my computer builder who designed most of the systems many coal plants use here in SW Pennsylvania... unless you actually work for Microsoft, I'm going to take this guys 25+ years of expertise over anyone here.