I've been running 4 and 5 characters with pretty good results. I only have 2gb of RAM, nforce 680i sli montherboard, 8800GTS video card w/ 320mb. I also run 3 SATA hard drives, running XP.

I noticed a definite load time improvement when I spread my wow accounts out across the hard drives. I leave my main on my C: drive, 2 on the D: drive and 2 on the E: drive.

before I moved them around, when I would take a zep trip across continents in Azeroth, it would sit there on "travel" screen (on all characters) for so long, that I'd end up getting off just as the zepplin left again to return...