As you can tell from the replies. 5-boxing isn't 5-boxing, isn't 5-boxing. That is to say, 5-boxing in WOW isn't the same as 5-boxing in RIFT, which isn't the same as 5-boxing in EQ, etc.

Based on your signature, I think we're assuming that you're going to 5-box WOW still, not move to 5-boxing RIFT or LOTRO or some other more modern game. From my experience a Core 2 Quad is sufficient for most previous generation games, but I can only 3-box RIFT on that, and it still doesn't run silky smooth.