I've been running two accounts on two PC's (one my older XP system and one my newer Vista system) for quite a while.
Prior to this, both of my systems were XP based. Up until now, the last few years has been two WoW accounts.

Prior to this, I was at five Everquent accounts on five boxes at one point; good old Windows 98 second edition.

I've recently invested in three additional accounts for a total of five. Warcraft runs smoothly except for Stormwind outside of the AH, which is choppy even with only one account going. BG's/Raids, IF and Shatt are smooth, so I don't understand this.

I'm running Vista 64-Bit with 8 GB of Ram, an overclocked Q6600 and 8800 GTX; my system handles five accounts without issue.
64-Bit Vista can use up to 8 GB, which is a lot more then XP, although in retrospect I don't need as much ram as I went with.