Andy:
I thought you were doing a budget build and have a preference on the AMD processor so I didn't comment earlier.
I have recently built 2 desktops with Intel i7-930. With 6GB memory. Each is a little over $1000, not including the Windows OS.
Here is the thread where you can find the details on the part numbers etc., and some details on how to build the box:
http://www.elitetrader.com/vb/showthread.php?s=&threadid=195662
The MoBo I used has 3 PCIe x16, 1 PCIe x1 and 2 PCI (the old one). I plugged in 3 x PCIe x16 dual-head display cards (PNY Technologies). No problem. Drives 6 monitors right away. But when I tried to use a couple of the old PCI dual-head display cards, FX5200, I ran into problems. I didn't see FX5200 being supported in Windows 7. I didn't want to spend time to mess with the issue. I ended up buying 2 of the "SIIG USB to VGA" adapters, about $50 each, to drive the extra 2 monitors. So on one of these boxes, I am running 8 monitors on Win7 and i7-930 with 6GB RAM. No problem so far. In an earlier post you worried about compatibility... there you go. One example that worked.
Since I had the impression that you are on a budget in building this new PC, I wouldn't think you would consider the quad cards. Because they are priced higher than a dual for sure. The one cited upthread has a list price of $400+. All the dual head cards I bought (PNY Technologies, SVGA) were about $50-$60 when I pick them up on sale. $100 to drive 4 monitors instead of $400. And you need 2 of them. The price of the 2 quad cards is about the same as your PC itself.
I am using a 600W power supply. I think 1000W is an overkill. I used to have an older generic HP box with a 250W and had 3 dual-head cards in there for years with no issue. But the i7-930 processor does seem to draw a lot more power but I think ~600W is enough.
But you are likely to find challenges trying to keep your old PCI based cards and making them work with the new cards, especially if you have a mix of different brands. It's the best to stick with the same make on these display cards.
Good luck!