That was the reasoning here also. The current thought train is that a high storage card combined with seperate images (such as JPGs) will drain the fastest, high storage with PDFs (just as big, just as much power to haul out and display but MUCH less indexing) next fastest. Text seems to be the lowest drain since it's not hitting any stoarge very much combined with....well, the fact that that is really what it is built to do. Display text. One good sized image can easily be the size of a book in terms of storage and is a bit harder on the battery.
Next goal is a 2gig card and see if that makes it worse. Ah, science....
(we're getting quite the standard sheet here between in club testing and MobileRead)