I have a 32 in my Evo and its been perfect. Do you really need anything bigger at this point? I'm sure the newer generation of phones will support the 64 but I don't think the Evo does.
Considering I only had the stock 8gb that I recently filled after 2 years lol, I purchasd a 32gb and its good to go...plenty of room to spare for now...I take tons of pics
***Disregard any and all typos as my large thumbs cannot fit the keyboard correctly***