RAM is very important yes, but for most cases the biggest improvement you can make for boxing is your harddrive.

Even a high speed raptor hard drive isnt really good enough for multiboxing. SS drives are something like 100 times faster than the fastest traditional raptor drive. (dont quote me on that though, cant remember exactly the speed difference)

I'd choose a priority system like this:
Get 8 gigs ram > get 64 bit windows > get solid state drives > get 12 gig ram