Hi,
I agre with Eric's comments. I have a 5d2 which I find to be much better than my original 5d -- mainly because it does not collect dust, my 5d loved eating dust! I would not consider a 5d (mark 1) given the real advantages of dust reduction, live view, larger files... of the 5d2. Oh yes, allegedly the video capture is great, but i never use itjavascript:void(0);
Since you own a Nikon, and one or more Nikon lenses I presume, you should consider staying with that brand. That way you already have a backup camera and lenses.
If you find that a Canon fits better in your hand, save your money until you can buy a new or used 5d2, lenses, batteries, and so on. Until then use what you have.
Jean-Michel