The specs say up to 32 GB.   Class 2 is fine.  The Class refers only to the write speed, not the read speed.  I use a 16 GB Class 2 card on my Tilt2, and it plays music and video just fine.  I upgraded from a 6GB Class 4, and have not noticed any difference in read speed.  Writing large files to the card will be slower on a Class 2 versus higher classes, but not a big issue for most people.  I can see how the class may possibly be an issue for phone that record HD video (where a high write speed may be neccessary).  But for the Aria, the higher Class card are probably not worth the $$$ (and have been hard to find lately).