Hey Maz, welcome! I'm rather new too and this place is definitely a great avenue for a wealth of information from dualboxing to 5-boxing it. I'm a five boxer with a similar setup to yours. I'm running my main on a AMD Athlon 3700+ 2.2 (clocked to 2.4), 2GB RAM, Nvidia 7900GT 256M on a 22" monitor. I refused to sacrifice my 22" amongst 4 other characters so they're sitting on my laptop which is very similar to yours but with only 2GB RAM. The laptop does pump out some heat but so far with all the video setting turned down I get decent FPS and little to no lag. Your laptop would probably be a better bet I would think but if you're hooked on using your desktop, why not try it out and see how it runs. I had to play with the settings for my setup but eventually it all works out the way you want it too.

As for what toons to run, I have two sets I'm currently running. 1x BE Pally, 3X BE Mage and another set Orc Warlock, Tauren Druid, Undead Priest, BE Pally, Troll Mage. I have noticed that the Pally/Mage quad group is easier with only two classes and the in sync casting is awesome looking but the 5 different classes is a nice challenge. I'm more of a PVE kinda girl so I'm planning on destroying instances but I'd say if you're into PVP go with 4x DPS, 1x Healer or something like that. At least that seems like the general consensus here or else go for 5x shaman b/c they seem to destroy in PVP and PVE (even though I've read 5x hunters are insane too!).

I also plan on having a 5x druid team so I can spec them for PVP or PVE. Whatever you choose, as long as you enjoy playing it, that's all that matters. Good luck with leveling!