If you want 720p, 1080i or 1080p support there's not a cheap way. In fact, the only reasonable solution I know of (and this is what IGN and the other gaming Web sites use) is by purchasing an Infinity card from Black Magic, which would require a Mac Pro or a PC.
You need component or HDMI HD input, which is sort of rare and requires lots of bandwidth across the bus.
http://www.blackmagic-design.com/products/intensity/
If you just want 480i there are several component/composite to Firewire 400/USB 2.0 devices on the market. The composite to USB 2.0 are the cheapest and I would recommend El Gato's Eye TV Hybrid.