The first big bad question in these matters is always about budget. If you have the money for a Canon 20D or a Nikon D200, I wouldn't event consider the D50. Aside from choosing a body, though, you'll want to carefully look at the lenses each company makes and decide what is right for you because that's where the big investment is. Camera bodies will come and go every 2 years or so, but your glass should stay with you much longer than that.

As far as narrowing the brands down, I've never had any experience with Olympus DSLRs so I don't have a valid opinion, but I would imagine there is a reason that almost every pro out there uses Canon or Nikon (lens quality being a big factor).

If its a toss up between Canon and Nikon, in all honesty they both make such great products that you can't really declare one better than the other. Again, figure out if you'd rather invest in Canon or Nikon glass. After that, I'd say compare the 20D (or 30D) vs the D200, or the Rebel XT vs the D70.

Nikon comparison chart: http://www.nikonusa.com/fileuploads/...art_060530.pdf

Canon comparison chart (its a little out of date):
http://www.usa.canon.com/consumer/co...categoryid=111