I've heard that above class 4 or 6 is pointless for phones and only good for real cameras. I've heard some people had problems with Class 10 on DX. I got 8 GB class 4 myself because I just wanted to put a few games on it.
i bought a Transcend 32 GB micro sdhc card off newegg.com for like $54 a few weeks ago and haven't had any problems... Make sure you don't buy one off ebay or are extremely careful... I bought a generic 32 GB sd card from ebay and it turned out to be fake (Only held like 2-4 GB but I was able to get a full refund so all was good)
EDIT: I also remember reading(but can't prove that this is true) that some smart phones can't read class 10 cards...It may not have been for the Droid X...just something to keep in mind. A Class 4 or 6 would probably be adequate for your usage tho.
I can verify that class 10 does work in a DX, just not well. I didn't even think about it when I replaced my dead card (six months old and it died, WTF) with a 16 GB class 10 PNY. At random times apps will claim the card doesn't exist, even if I unmount then remount it; sometimes it takes a full reboot. Pretty annoyed at myself; I plan on picking up a class 4 or 6 SanDisk to replace it.
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