The A80 is a great camera. It has the slower Digic I in it, but as you already use this camera you will be used to it and won't notice! I had and used the A70/A75s for a long while - would still be using them if some nice person hadn't flooded the last one. I picked up a cheap A520 to use...I haven't had a chance to get it underwater yet but the only downside to that model is two AA batteries instead of 4.
If you look at the pink link in my sig and scroll down a bit you will find the A80 along with a link to some photos (actuallly I think it's the A85, but same dog) taken with internal flash only.
I wouldn't worry too much about MP really - 4 and above will be fine.
Going with the camera you already have will save you some bucks, but if you think you would like the more MP and/or the newer technology then maybe check out some of Canon's newer models. Keep in mind that with your curren A80 you can simply add things like wetmate lenses to your housing. If you go with the A620, a great camera, you will need to head to the Ikelite housings in order to be able to put wet mate lenses on. Again, the pink link talks a bit about extras you may want to add.
Have fun shopping!