I wouldn't say it would box really well.
But it would certainly box.

I was running a Q6600 (stock speed).
8 GB of DDR2.
4870X2 (but have moved to a GTX275 because of conflicts).
10k rpm Raptor drive.

Dalaran was very choppy, for 5 clients.
Wintergrasp as well.
Would sometimes lose follow, on epic ground mounts in battlegrounds.
Depending on the number of toons in the area, and the amount of stuff going on.

The game was definitely playable.
Instances and other cities besides Dalaran were fine.
Unmounted on the ground, all battlegrounds were fine.

I've upgraded to a much better system.
At least for 5-boxing wow.
I probably don't need what I have for boxing.
But it is smooth everywhere now.
Except Dalaran is still Dalaran, but cannot lose follow while mounted.