There are some great deals on 32 GB SanDisk Class 2 micro SD cards on Amazon right now. They are about twice the price of a 16 gig card, which makes me feel bummed since I bought mine at a good deal from VZW 6 months ago for about $100.
SanDisk is the only brand I will ever bother with. Class 2 is all that you will ever need for your Droid X and coming from SanDisk, their ratings for their cards are far greater than any other brand. Meaning a SanDisk class 2 will usually outperform class 4