You want a SanDisk memory card, I think it's a "Duo Pro" or something. I'd say 1 Gig at least, 2 Gig would be better, 4 Gig would be whole bunches.
I bought an official Sony one and paid a bit too much, but it was all they had there at the time. I couldn't wait the day it would've taken to find the SanDisk elsewhere. 8 (