I think the last possible extra ram is worth it. My 5 WoW's were running on a 4 gb system, using 3.5 gb fulltime .... Then I added 4 gb more, now most of the times totall usage of the 5 wow's (and vista itself) is 5.5GB...
So, I don't know how stuff like this works, but it seems like the operating system capped my mem usage to a certain point ...

I run Vista 64 bit, and I would say go for that aswell, but I don't want to be a smartass ;