I would probably go for a class 4 or class 6 there are (as far as i know) 4 classes 2, 4, 6 and 10. You will need at least 4MB/sec write speed for 720p video recording when you get froyo. I dont think that class 10 is recognised by the desire but i could be wrong (and they are very expensive anyway)
I personally have a class 2 16gb pny card but i got it because lots of reviews stated that they were getting much faster speeds (5-6MB/sec) than the class rating. This is because the class rating is the guarenteed minimum transfer rate with high quality cards like sandisc etc you will probably get much faster than rated transfer speeds.
Btw mine was