I haven't done much looking around, but you could always get a couple of the 4 port pci capture cards. The 4 port model is just a bt878 chipset.
I have no idea what the 16 port model is hardware wise, but they only list it working on windows.
The PV-156 is precisely what you are looking for. It simply has 2 4 port bnc video connectors. However, they don't list it as for sale.... http://www.provideo.com.tw/pv-156.htm
some searches on google should reveal more hardware of this nature.
All else fails, check out the forums on zoneminder.com. I use zoneminder for the security camera setup at work. While it's sometimes a pain in the ass to get up and going... it works rather well.