I'd recommend at least 8 MB of memory. The Visor modules come with extra storage or memory. Fiddle with the contrast controls. Make sure you're satisfied. A backlight button on many models compensates for dimly lit environments.
Most handheld screens leave a lot to be desired when viewed in sunlight.
The exception is the Ipaq, which has the clearest screen I've ever seen. The Palm VIIx wireless eats AAA batteries if you spend a lot of time sending and receiving data. Look at the battery specs for the unit you buy. Once the novelty wears off, you may find you don't need all the bells and whistles that look so attractive in the store or on the Web. It has been my experience that the more separate functions a device offers, the more things can go wrong.