I have a Multibridge Pro which is an external box (and not this cheap), but there are PCIe cards as well. Used Intensity (HDMI only) cards are available for $100. You just need to check which of the Blackmagic
Cards will do live the Live Keying in SD and which ones in HD. As tested with a PS2 and PS3 component video works as well, so a Decklink card instead of an Intensity would work as well. The BMD cards use a powerful MJPEG codec which allow you to capture HD video without framedrops even on single-HDD systems (no Raid neccessary).
The Livekey function is a nice gimmick. You fire up a small windows (or Mac) small tool in which you load your overlay graphic (with alpha key channel). You can then adjust the density and the card will key the image over the input video signal and output it right away again.