I shot Velvia for years and after switching to digital I would never go back. If you shoot a decent amount, the money saved on film / processing pays for the digital body in no time.
Don't know about the 16MP comparison to velvia, I guess it depends on what you are doing with the slides. If you are going to make prints, then I've found that my 8MP 20D blows away velvia...well in terms of easily making quality prints up to 24"x36". It takes a while to get a process that works for you, but when you do you are off to the races. Getting good prints from slides is very tough and expensive. With my 20D I can easily make 24x36 prints that are tack sharp (assuming the original shot was made with care - same as film).
Now maybe you need 16MP to compete with velvia if you use a $100,000 drum scanner to scan the slide and pay $50-$80 for each scan and then waste time removing dust, but in the end I have found that for ease of making prints a decent 8MP digital blows away slide film.
The kicker with the 5D is that it is full frame and there is no conversion factor on the lens. If you are into landscape shooting and normally use and already own canon 17-40mm or a 24mm or similar wideangle lenses I would go with the 5D without even trying one out. It is that good. If you like to shoot wildlife and already own Canon lenses, then go with a 40D and take advantage of the 1.6x crop.
I shot velvia for 10+ years and there is no better time than now to jump into digital. If you are really into shooting, then do not hesitate.
- darren