It really depends on what you want to do with it and what spec your PC is. By far the best value for money is the Skystar2, this will cost you £59 or thereabouts from someone like www.sateuropa.co.uk (this is the latest more sensitive card, not all are). You can also get the Skystar2 USB and Skystar1 from the same source, the Skystar1 has its own video processor on board and has a video out, so better for distribution and use in lower spec PCs.
Hauppauge also sell the Nexus which is an excellent card, but like the SS1, is more expensive at £160 or thereabouts, I believe that this is one of the most popular units.
There are more cards available and some are identical to the ones I've mentioned above.