1) What are you going to be shooting?
Any lens back to 1977 by nikon can be used with the "F" mount digital cameras( D70 included).
2) Yeah, you can mount it. But you can't meter with it. And the dim viewfinder also makes it impossible to focus it manually.
Plus, you can mount nikon, m42 and some other funny mounts on the EF Canon mount. So that's not a plus for nikon.
Also you're looking at digital. Nikon is encrypting his RAW format, so you'll only have nikon software to work with. Canon is not doing that at least now. So you'll be able to use third party raw converters, which are really important part of the pro workflow usually.
20D noise handling is way better than that of noink.ISO 400 is noticeable already.
Also D70 doesn't have ISO 100, D20 does. This comes in handy when shooting outside with a flash.
There's a grip for D20 if that interests. No grip for D70.
EF mount is more versatile. It's wider and closer to the focus plane, thus giving designers more options to design weird glass.
Higher FPS and higher writing speed of 20D. Way better autofocus.
I personally find the D70 viewfinder confusing. The areas don't light up clearly... (very subjective though)
All and all, there are many reasons why 20D costs almost twice as much as D70. There's almost as twice as much camera in there.
