7600GT is about 50% faster than a 6600GT at reference clocks ... Of course, most 7600GTs are pre-overclocked, so most are about 60% faster ...
6600GT (Reference)
500 MHz x 8 Pipelines = 4 GPixels/s
1000 MHz x 128-bit = 16 GB/s
7600GT (Reference)
560 MHz x 12 pipelines = 6.72 GPixels/s (68% Higher Fill Rate)
1400 MHz x 128-bit = 22.4 GB/s (40% More Memory Bandwidth)
7600GT (XXX Edition)
590 MHz x 12 pipelines = 7.08 GPixels/s (77% Higher Fill Rate)
1600 MHz x 128-bit = 25.6 GB/s (60% More Memory Bandwidth)
The HDTV output means it can output Composite, S-Video, or Component video to a TV. Component video can output up to 1080i or 720p. Keep in mind that unless you have an LCD TV, you're not going to get good image quality with anything but movies and games from a computer.