The amount of data (MB/s) you need to write is: # channels * bit rate * (1 byte/8 bits) * sampling frequency * (1 kB/1024 bytes) * (1 MB/1024 kB). So at 24 bit 96 kHz, it would be (2 * 24 * 0.125 * 96000)/(1024^2) ~ 0.55 MB/s. A class 2 card is rated at 2 MB/s, so that should be more than sufficient. I guess the card might not perform to specification, but the margin there is pretty wide (assuming I did the math right!)...
I recently ordered a class 4 microSD for my PCM-M10 but they sent a class 2 (even though the picture on the web clearly showed class 4, but don't get me started). I was going to return it, but I figured I should just keep it for my phone and not deal with the hassle for a $20 item. I tried it in the recorder, though, and it worked great (although at 24/48).
I have also heard that slower cards may be better for recording audio, but I have no idea if that is true...