Hammersley: I would recommend the 20D based on my experience. Sell your current non-L lenses, and get yourself some L-glass. I currently have: 20D; 70-200 f/4L; 24-70 f/2. 8L; 1. 4 teleconvertor; Battery grip with extra battery, and (2) 512 meg cards. I went to Daytona 500 and Disney World and on those 2 cards I had over 2000 images. Most of the shots were experimentation and that is why I had so many of them. The 20D, battery grip, and 24-70 is pretty heavy but I was surprised at how easy it was to steady the shot. I have quite a bit invested and I know money is precious. You can get the 17-40 f/4L & 70-200 f/4L for about 550 each and that would cover most photographic situations. They talk of the cropping factor and how a 200mm is actually a 320, but in real usage a 200 is a 200. I have been going to Daytona for most of the last 20 years and early on I thought a 400mm would be ideal for the track and after using the 70-200 with the 1. 4x(which works out to 280), I still think a 400 f/5. 6L(or maybe the 100-400 f/4. 5-5. 6L) would give me the long distance shots I desire.
I agonized over the D70 and 20D debate, but I really wanted the extra frames per second(just awesome for action shots) and the choice of Canon lenses seem to be better and after bouncing the pros and cons off a friend of mine, he too went for the 20D. If you go for the 20D, I say get the 70-200 f/4L... . it gets raves everywhere and relatively cheap for an L lens. Compare it your other lenses... especially at the extremes and I think you will be wanting more L's.
My previous equipment was average and my results were average to a bit above average, but usually nothing that would surprise and shock me. I use mine mostly for action shots: auto racing and airshows with some family shots in between. I am not big on the use of flash(especially in public) and the 20D allows me to get shots I never thought possible before. I can take non-fuzzy pics as low as 1/20th of a sec and that is without "IS".
Just my unbiased opinion. Email if you have any more ???? as I will answer all I can.
N Dennis