If you already own Nikon lenses, than I would go with Nikon or vice versa if you already have Canon glass. As far as which body, solely depends on budget and what you generally shoot. For general photography the Nikon D80/Canon 50D or XTSi are good choices. If you plan on doing more scenic (aka landscape) than you want a full frame Canon 5D or Nikon D700, both fairly expensive bodies, but they don't have a crop factor (Nikon is 1.5x & Canon 1.6x).

I usually point anyone that asks toward either a Canon or Nikon system, simply because they have the largest selection of lenses and support gear. Easier to find used equipment and generally good 2nd party lenses for both camera systems.