The only cards offering hardware acceleration for Premiere are the Matrox Axio and RTX2. They will only work with PCs at this time.
There are many suppliers of I/O capture cards for both platforms, including Aja, BMD, and Bluefish, but these cards don’t accelerate anything. Some models do support hardware up/down conversion in real time.
I believe there are issues with drivers for these cards being available for Premiere on the Mac, especially if you intend to run FCP on the same box. But I’m sure they are working on it.