I'm having trouble finding specific recommendations myself, though I'll probably end up with a stock/CM7 dual boot from internal memory and using the card for shared storage.
The issue as I understand it is that the 'class' of SD card refers primarily to sequential write speed, which is the most important benchmark for use in digital cameras (and I think for a storage drive in general). The key benchmark for an Android system drive, however, is the speed of small, random writes. Manufacturers don't seem to be optimizing or even measuring this benchmark, nor is it clear what constitutes a good, adequate or poor speed for use with the Nook.
If anyone has assembled these benchmarks and/or compared them to actual performance on the NC, I haven't found it yet.