1. Dual boxing is more like running one toon, with a little extra firepower - or a built-in healer or aggro reducer. (I used a paladin on /follow, and just had them cast buffs on my main for a long time, but I've found 2 dpsers are far more efficient.) 5-boxing will drive you nuts if you do many collection quests. But with 2 toons you can't instance or otherwise do much more than you could with one. 5-boxing opens the door for instance runs.

2. Paladin. Paladin. Paladin. Multiboxing is more complex already if you are performing tanking, healing and dpsing all yourself. My dpsers can't go all out as it is now with (better geared) other live warriors and druid tanks. But with my own Paladin tank I can hit instances with few worries. If you plan to 4/5 box and hit instances, you'll likely want a tank. On the other hand, those running 5 shammies do it somehow...

3. Personally I like a druid healer better, but chain heal certainly works nicely for a multiboxer.

4. In my dps 4-pack I've found the moonkin druid often runs out of mana. The shaman and mage almost never do. Personally, I'd take an extra shaman over a shadow priest, but that's me.

5. You don't mention if the cpu is single core, dual, triple or quad. While a single core *can* handle multiple instances, the multi-core processors will run them better. (The ideal being one core per instance of WoW running.)