Five boxing isn't necessarily harder than three boxing. It certainly is different and there is a bit more micro management invovled but I'm sure you will be able to handle that just fine.

While I'm no hardware expert I'm pretty sure it will work just fine (I five box on a three year old laptop hehe)

As far as end game 5 man instances go, a good setup can always be found with the Tank/healer/three dps teams. I myself roll with a Pally/Holy Priest/Three mage's. Seems to work well with all the CC and the ability to AoE tank.

Personal opinion: 5 boxing is so worth the money