http://www.aegismicro.com/products/pt_vp101.htm
Check that out. I used to install these systems and they worked pretty well. I recommend an Intel motherboard as those seem to be pretty compatible with these. Need IDE ports - and you need IDE ports for HD's or need to do some research to make sure your RAID will work with this card.
Going to be hard to put something together for $1200 honestly. Good cameras are pricey. Panasonic and Pelco make good cameras. There are others that are less expensive, but are not as good quality.
If you already have a computer, you might be able to put it together for that much. But that's the ONLY thing this computer will do as the chip boots to a linux distro and brings up their software.