Ram is the issue here. I run a quadcore 2.8 with 6gb ram and 5wows. I am about on the borderline as far as RAM distrubution goes (for hanging out in high density areas anyway - everything is liquid except shatt), in fact I'm upping it to 8gb soon to make everything fluid for the 2 clients that share a processor (its usually those guys who break follow cause of video lag)

So ya I would say 4gb is prolly enough for instances and outdoor - but high demand areas need more ram for wow - and in wrath each client will need even more.