I'll add my 2¢ worth in here as I have also been trying to work out what capture card setup to use.

An alternative to those cards mentioned by Moth3r is the MCE TV capture card.

http://www.geeks.com/details.asp?invtid=MCE-TV-7135-PCI&cat=VID

It's main advantage is it's price - dirt cheap!

This card uses a newer Philips chip than the PDI/Sweetspot cards. I have no idea if the comb filter inside is more advanced or if that will make any noticable difference to the end result. The Philips chips used in the MCE/PDI cards both have 'Adaptive 2/4 line comb filters' as mentioned in their respective datasheets.

The only thing that I can see that the PDI has over the cheap MCE is that the PDI also uses the BT878A 'Fusion' chip. As mentioned by Moth3r here, there is a special BT878 tweaker within Virtualdub which will probably enable better results from the PDI as compared to the MCE.

Neither the PDI or the MCE have BNC connectors, but if you are pretty handy with a soldering iron then BNC connectors probably won't be too difficult to fit.