There are a few of those gadgets in the pro-sumer range, but as you suspect you'll need something more commercial. You're likely going to end up looking at a 4 or 8 channel PCI-E card.
The big question though is resolution - are you encoding an HD source, or an standard definition NTSC/PAL source? For the later there are plenty of solutions on the market, most targeted at surveillance systems and with a healthy price range (sub $1000 and even sub $500). For real time HD the technology does exist (there are several 1 chip solutions that can do full 1080p encoding with the lag time measured at just a few dozen lines), but will usually cost you more.
You're concerns about "playing well with others" will also continue even with commercial solutions. Remember that many of these systems are made to work out of the box, and there isn't a really good PCI/USB standard for sending pre-encoded video.
An alternative where there is already a good standard in place is IP cameras - or specifically look for an "analogue camera to IP camera" conversion box (also called a Video Server). Basically it's a box about the size of a cable modem (or larger for models supporting a lot of channels), that converts the input from a non-IP camera (or other video source) into H.264 and sends it off through the net just like an IP camera.
As an example here is a manufacturer (note that I just Google'd them up, so I can't give any recommendation for their particular products)
