Welcome to the forums. Nice to see new boxers, means this community is heading in the right direction.

1st - Are you using Vista or XP or something else?
2nd - Is that a total of 2048mb of Ram or 3 sticks of 2048mb?
3rd - If answer to nr1 was XP make sure you have at least 4mb of Ram
4th - If answer to nr1 was Vista, I'd chuck in as much as possible to up your Ram.
5th - IMO Keyclone is a good way to start.

Additions - Multiboxing is about personal choice, there are many here who have prved that 1 Paladin and 4 Shamans can do All Heroics, some have proved 5 Shammies can do all Heroics. Its down to what you want and what you think will be good. There is nothing stopping you trying 5 rogues or 4 pallies and a resto druid. :P

There are some setups that will make your life easy, some that will challenge you and test you, depends if you want tried and tested or OMG 5 rogues own!

It can be hard sometimes and you will have down points where you think wtf am I doing this for. Look further than that and imagine the pwnage you will have at level 70 doing your own heroics or farming those arena's like Ellay. I can't wait to see how easy 5 hunters will do at farming stuff. I mean, thats nonstop grinding, forever. Pets, auto shot X5! You cant beat that. gogo Gold Income!

Other than that I think your setup is fine, I play with a 22" and that does me fine, with Keyclone and PiP ofc.

Need anything, shout up or drop a PM and I'll try my best.

Khayos