Doesn't matter, I think I found it.
It seems you are better off buying the 1 gig card. According to Wikipedia, the Type M is a new technology that allows manufatcure of much larger storage sizes (up to 8 Gb), but as the card in question is only 512 Mb, there's no benefit there! Also according to Wikipedia, the type M has the drawback that it is much slower at reading and writing, so, as I said, you'd be better off with the faster, cheaper, larger 1 Gig!
http://en.wikipedia.org/wiki/XD-Picture_Card