If you can swing it, check
SVS' outlet and get 3 Ultra bookshelf speakers. Their B Stock has been very good to me and the Ultras show up in the outlet fairly often for $399 apiece.
I've never heard the prime speakers, but have read lots of good about them too. I love my Ultras though. Even starting with just 2 of them and adding a center later would be perfectly acceptable and add the center when you can afford it.
The typical center channel design is more for aesthetics and having room to place it. A matching bookshelf will always sound better.