Beyond getting a dslr, it's mostly a matter of preference and budget. Sure, I'd love to me shooting with a new 1Ds Mark III, but that's out of my budget right now. Both of my cameras are more than capable of doing what I need them to. Without the lenses though, I'd be screwed sometimes. Not all places allow flash and/or have decent light, so an arsenal of good lenses is a must, IMO.
Canon and Nikon are the 2 main players. I easily narrowed it down to the 2, simply based on the lenses and flashes and other stuff available. After that, Canon just felt more natural to me. IMO, Nikon's are set up weird. I've heard Nikonians say the say about Canons though. Go to the store and hold and play with your top choices.
My list of must bring wedding goodies right now is:
40D with grip
20D
580ex II flash
lightsphere
several sets of batteries, for cameras and flash
around 30 gb of memory
tamron 28-75mm 2.8 lens
sigma 70-200mm 2.8 lens (buying next month, renting for one more wedding, which will pay for the lens)
50mm 1.8 lens
I also have my computer with cs2, lightroom, and an external harddrive for storage
Still on my list of things to have:
100mm 2.8 macro lens, for details and ring shots (I LOVE this lens)
a fast wide abgle lens
50mm 1.4 lens
back-up flash