If you are getting the Primus 362s get the matching center - the C350 - and a pair of P162s for surrounds.
This is in fact the exact setup I was looking at.
It will sound much fuller if you go with a sub rather than a center. Don't go with a dinky cheap sub either, as the p362's have pretty good extension, and a small sub isn't going to be much of an improvement. One I would suggest right now is the HSU VTF3.3, which is on sale right now, 10% off regular price. Incidentally I am setting up a p362/VTF3.3 combo for my mom and dad right now.
Other subs I might recommend, which will fit your budget, are the Elemental Designs A3-300, if you can stand the wait, or the SVS PB-12, or the Outlaw Audio LFM-1 EX, which is sort of a VTF3 clone.
I dont have a problem with dropping that much on a sub, but I would not be able to do it right now. If I were to go with a $500-$600 sub (is it really worth it by the way, over a ~$300 sub?), I would have to get the p362s and C350 now, and the sub and surrounds later when I can afford them.
So NOW it's really a question of long term vs short term gratification.
short term, I'd get the fronts + cheaper sub.
long term it would be fronts + center, expensive sub later.
I think I should probably go with whats better in the long run.