The more the megapixels the more storage that requires. Two things to consider when looking at this is how big is your hard drive and how big of a memory card is in your camera. The card that comes with the camera is never big enough. If you get a 3-4 megapixel you want at least a 64 meg card, a 5-6 a 128, 7-8 256 meg card. A 256 card can hold about 60 pictures on a 7 megapixel camera, I am guessing. I have a 7.2 megapixel with a 512 card and it holds 129 pictures.
For the bells and whistles, some that are cool to consider are,
night vision..some cameras, the nicer ones have night framing that make black and whites out of night shots. Also, camera that are threaded for an extrernal flash or for a larger lens. The size of the LCD screen doesn't matter so much cause when you take pictures, you are going to probaby use the tradional view screen (as opposed to the lcd) because putting the camera up to your face steadys it for clearer shots. Your eye socket gives the camera a good base. Some digital cameras are starting to have shutter speeds which helps with picking up motion without a blur. The higher end ones can make really short mpegs, meaning film clips, on the camera.
batteries life is important to look at too but the technology varies a bit. Some of the basic models take double AA lithuim ion batteries whereas the nicer ones have built in recargable packs that last for a few hours at a time. If you post model numbers you are considering mabye we could give you feedback.
Sava's idea is good. Sony makes really high end digitial cameras. So does, Canon, fuji, HP, and kodak.