Yeah, the HD5670 is the lowest card that can effectively do vector adaptive deinterlacing with smooth playback enabled. This is only really necessary for interlaced content, especially if that content has diagonal lines (think white lines on a football field at 1080i). Progressive sources won't make any difference.
If the OP does go for an HD5670, the HIS HD5670 with the IceQ heatsink should be at the top of the list since it is dead quiet.