I was at Circuit City yesterday and I saw that they had a two pack of SanDisk 2GB MS Pro Duos for $30. That's a pretty good deal on 4GB of memory, if you don't mind having two sticks.
From what I know 8 is the largest at the moment but the read/write speeds are much slower than the 4 gig. IIRC, the price is quite high too in comparison.