8800GTS 512MB? Those are $200. Should do you nicely. If you need to upgrade your power supply, get an 8800GT, and a new PSU. Those two together should run you ~$250.
Do you use vista? If yes, how does the TV Tuner show up in the hardware manager? If it shows up as a display adapter, then you need to get a 3870. Vista requires all display adapters to use the same driver, so you can't mix and match ATI and Nvidia cards like you used to be able to. I don't have a TV tuner, so I'm not sure how it works.