It's worth it to buy a 24" computer monitor, don't try to use an HDTV as a monitor. It's worth it to use an internal video card with CATV inputs instead of a USB device. Your video card should be PCI 16X, with at least 500 mb memory and on board GPU. Adding this to an older computer with single core processor will result in sub-standard HDTV performance, which is just a waste of money. (video freezing, jittering, unable to watch full screen)
Many have tried to add video upgrade to an older computer and realized that it was a mistake. What you really want is a dual or quad core processor, preferably quad core. The older single core processors cannot handle the amount of information.