Are you going to use it as just a big monitor or to game on or for movies? If you are going to use it as a big monitor just imagine it as such. If you want to watch movies or something like that you don't need much, treat it like a HTPC.
If you are using it as a HTPC or just a big monitor, I would get a 4650 or 4670. They have build in sound and will pass it through to HDMI which will be nice for movies or streaming media.
If you want it for games, get a gaming card like any other monitor.