depends, what I/O do you need?
the decklink cards are very cheap if you only need HD-SDI in/out (or HD-SDI in/out and component out).. the Kona LH is good if you need all digital and analog in and out.. the Kona3 and Decklink Multibridge are great (and do nearly everything, including upconverting) if you have the money.
the cards are getting pretty cheap, but does anybody know of an affordable HD monitoring solution with proper color?
++ christoph ++