if you are keeping your current quad card, and assuming your new PC comes with a video card, just make sure it is a dual (one came stock on my dell 8400). i use the dell card and a matrox quad and am good to go.
3 nvidia dual head cards are the way to go. If a qual craps out you are out big bucks for what it is........nvidia is microsoft compliant configured so much easier for upgrades from windows update......................Matrox is not, NUFF said about that.
Quadro should be with another quadro card, do not mix different brands. Dual-View is from nvidia and i found it was much better than matrox for setting up different monitors. and i have used 6 monitors for charts with 3 nvidia cards trouble free for about 5 years now.
next pc, is going to need pci-x cards so 3 dual heads are cheaper than a quad and a dual. Plus with dual cards your options are better to configurations.
Again, matrox is last choice, nvidia young man, go with the cards that the BIG trading houses use in the fancy trading rooms.............