8GB of ram is plenty for 5-boxing or less.

I'd keep the video cards as non SLI/Crossfire.

An SSD for the gaming folder is nice.
It will speed up your play, any time things are loading.
Such as entering the world, or zoning on a Zeppelin/Boat, or in general in a city as others are constantly moving into range and you're loading their textures for armor/weapons.
An SSD won't help you that much once inside a 5-man instance, as there won't be a lot of loading going on.




You have two existing accounts... A and B.
You can either use both, or just one of them.
If you use both, set up two RAF chains where your best "booster" in on A.
A > C > D
B > E
You will use A to boost new toons on BCDE.

Alternatively, use just one of the accounts.
Whichever has your best booster will be "A".
And go with two chains:
A > B > C
A > D > E
This is good, if you're starting on a new server and have transferred one toon there.
Or its good if you're starting to box from scratch, in which case you'd level five toons initially with the "A" toon being a good booster for later teams.

If the extra $$$ to buy another account is not that big of a deal, I'd go with the second method.
It will make starting on a new server or faction (or both), much easier...
Plus you can transfer toons from the existing account that you're not boosting/RAFing with... they're not automatically discarded.