To be honest you won't get better than you telly...though in fairness, TV capture cards do a good job: it's hard to tell the difference.
Generally speaking,
Pinnacle Systems have very highly regarded products. I bought one a couple of years ago, after lengthy searching and thinking. They did fall down on WinME support though, but I believe that has been addressed.
Basically you have a choice between PCI and USB versions. So cut your cloth so to speak. I should imagine a PCI card would suit the setup you describe. You can use a regular signal booster if necessary, and try to keep the cable as short as practical.
Graphics card wise, there are (were as at June 2002) no programs in the offing that even approached needing more than 64MB of video RAM, so don't waste the wonga if you don't have it to spare. Also be aware of the differences in graphics chipsets. An hour spent at Anandtech.com or Tomshardware.com would be well spent. Bottom line though is the graphics card doesn't play all that much of a role in picture quality... it's the tuner. Of course, digital TV is a different ball game and I can only share analogue experience.
Hope this is of some help.