WRT to point 1.
With the current deals going on with the D70s - they are bundling a Manfrotto 190 tripod and head, two CF cards, etc. If you can use the extras, the D70s actually is not a bad buy at the moment.
WRT to point 5.
While I find the WB on the Canons to be better than the Nikons and the edge on overall image quality going to the Canons, I'd still buy a Nikon at the low end of things. If you want to buy into Canon, start with the 30D.
This is bec if nothing has changed between the "old" 350D and 400D in terms of User Interface, I'd go with the Nikons as the UI on the low end Canons are crappy IMO. Compare the UI of the 350D with the Nikons to know what I mean.
Also the cheap and cheesy feel of the 350D isn't to my liking. From what I have read so far, this hasn't changed with the 400D. The Nikons are much nicer in this department.
With the Nikons, I prefer the D70s just because it has a dedicated dial for aperture (or was that shutter speed - can't really remember now) and the fact that it takes CF cards (I find SD cards a tad fragile and small enough that they can get lost easily), but you might like the D50. Try before you buy.
No experience with the D80 as yet so I'll reserve my comments on it. But the 18-135 kit lens that comes with it apparently has a plastic mount which kills it for me.
Best thing to do is to get your hands on all three or four bodies and test them out yrself. Use the advice given here as a general guide. Check out other reviews on the net to supplement what you have already been told.