I wouldn't recommend anything other than 8gigs of fast DDR2/DDR3 RAM for 5 boxing on one machine. 4gigs will work if it's fast, but you'll still lag a bit in Shattrath with only 4gigs. This means you'll need vista64bit to be able to use all of the ram and a decent motherboard as well.

If you don't want to go that route, than just deal with it or set up a macro that completely minimizes your sound and video settings. For example, setting max/min frames per second for the main and the background in keyclone or the console manually, turning off all sounds on your clones, and reducing the video settings on all of your clones along with possibly your main.

Your video card is ok, not great, but it's not your problem at all. Your processor is perfectly fine. You didn't list your motherboard so I can judge it's performance. Your ram is not on par with what I'd use to box 5 efficiently. What resolution are you trying to play with in each window, and how many monitors are you using. More info?