If the budget doesn't scare you, then the next step is to pick your religion (brand). It truly is a religious war between the two big brands (Canon vs Nikon). This isn't the best place for that debate, but if you want extreme biases you can head over to dpreview.com's forums for either brand.
Personally, I am biased towards Canon. I have a Canon 20D, while its a few years old it is still a solid camera. When you pick a camera brand, think of it as picking the lenses...as the bodies will come and go, its the lenses that stay. I like Canon glass, I love the red pin striped L I use primarily...though it won't fit in a underwater housing.
I would suggest finding a local shop that carries both. Go in, play with the Nikon and Canon equals. Find the one you like the best, the interface, how it feels in your hand. Beyond that, they are pretty equal. Both take excellent photos. Both have great in-brand lenses, as well as 3rd party.
When it comes to the underwater side, its all pretty equal. In each tier the housings are about the same, and the lens ports are the same. Some brands of UW housings may not cover all of the camera models, so maybe that can be taken into consideration.
I would suggest deciding between the 2 "entry" tiers. Look at the Canon 400D Rebel XTi and the brand new 40D (*drool*). Then look at the Nikon D300, D40(x) or N80. Any digital SLR you buy from Canon or Nikon will be a good choice.
Keep in mind, any SLR system is a endless can of worms. I guess you already have the diving can, so why not added another. I am currently working on syncing the two together, might as well *really* start spending money. There are endless sources of information and misinformation on lenses and brands, but the dpreview forums are always good start.
Of course, I haven't even taken a single picture submerged yet. Though I have about 6200 clicks on my 20D. Oh, make sure you have some place to store the several GB of pictures you will collect in no time at all. Find your favorite test subject (pets or kids, spouses don't appreciate it) and test away. Learn how each of your lenses reacts for depth of field at different apertures and zoom levels. Its all about DOF and exposure, oh and always shoot RAW
Good luck!