Most of the 5 megapixel cameras are now well enough under $800. You can't afford a dSLR for that price, so sticking to the newer generation digicam's is ideal...
Personally, after having tried Nikon, Olympus, and (admittedly a cheap one) Sony... I definitely prefer the nikon's. Especially if you want tank shots... the Nikon seems to have the closest focal range and best macro ability. I haven't tried any of the newer cameras though.
Check out dpreview.com for lists of features for each camera and their respective reviews.
