It supports 2GB SD memory cards, up to 32GB SDHC cards and up to 2TB SDXC cards. I am using a 32GB SDHC (class 10) card and a 32GB flash drive with no problems.
I'm using a 64GB SDXC card in mine, and it works fine - although I did have to buy a card reader that worked with SDXC cards, as the inbuilt card reader in my PC (which is quite old) wouldn't recognise the SDXC card but was fine with SD
I have 32GB card which works fine. I would say however, buy a class 10 card as it reads / writes faster. Go for a decent quality card as well. I recently binned a cheap class 4 card as it was slow and constantly needed reformatting.
Just one thing to mention - many iTunes tracks are in .m4a format. My system which is the basic Composition Color Touch won't read them. So converting to .mp3 now!!