I'm still knew to multi-boxing, and I'm having some annoying problems. I'm wondering if I have to get some upgrades to my system maybe?

This is what I'm running, and my goal is to get to 1 PC running 5 accounts.
Win XP Prof.
2 20' HD Flat Panels
Quad 2.66GHz
4GB RAM
ATI Radeon HD 2400 XT (Should be 512 but it says in the properties that it's 256)

Do any of you see a problem with this set-up that I don't know about? I'm wondering if I need a better or a second video card, and maybe upgrade to Vista. Any help would be very appreciated because I've hit a wall lol.