You can buy a complete system or you can make your own. I have built a very reliable one for about $550<assuming you use your existing PC>with four cameras running constantly. Of course it cost me about $1000 more to figure this all out but hopefully someone can benefit from my expensive experience.
Best camera for the price with high resolution and black&white nighttime IR in my opinion is the Swann Bulldog = http://www.swann.com.au/s/products/view/?product=310
Most reliable 120 fps card I have found is the IVC-200 it will do 30 fps per port about $200 (found it on ebay for less than $100 but haven't seen one on there in a while, OrbitMicro has them though for about $200) = http://www.ieiworld.com/en/product_IPC.asp?model=IVC-200G
The software I like is affordable and works pretty darn good. Blue Iris and it is $39 for 16 cameras. = http://www.blueirissoftware.com/index.html
Some say it is a CPU hog and on a PIII or smaller type PC with four cameras running it is, using about 80% of the machine. There are probably more efficient programs but Blue Iris is feature rich and I am using it on a new P4 machine so no issues and runs great. Plus the developer comes out with updates for it on a regular basis.
Hope this helps someone. -rhane