Your two matrox cards should be more than enough for you to run 6 monitors (each one is capable of 4). I'd say pick up a quad core Intel processor and a lot of RAM, say 8GB or so.
That being said, one of the 8-core bulldozer processors will have more than enough power for your multi-threaded applications, and be easier on the wallet, too.
The 8150 is on sale for $210 right now at NCIX,
and the
M5A97 is on sale for $95.99.
Round out the system with a
Crucial M4 256GB SSD on sale for $215
and some
Kingston 2x4GB RAM for $54
and you've basically got a complete system for under $600, plus powersupply and case.
I think you'd have a hard time finding a new intel platform with an SSD for that kind of money.