The Canon 300D really is a neat camera. I think the photos taken with it on this site speak for themselves. The 6.3MP is more than you will ever need and, more importantly, the smoothness/cleanliness and color is amazing. Large prints are no problem as with 75 dpi you can get 40-inch wide prints.
I shoot with the Sigma 70-300 APO, a pretty good budget lens. With good weather, the quality is sharp even at the 300mm end (this requires stopping down, though). Seeing as you are considering the EF
100-400L (just under $1000 new/used on ebay) you have nothing to worry. It produces astounding results. The only one downside with the 300D is that you can only shoot 4 frames at a time at 2.5 fps but the buffer empties out quickly so even when shooting moving aircraft, there is only a second of delay before you can get the next shot.
Overall, it is an easy camera to learn on and shares quite a bit in common with the 10D. Build quality is acceptable; not up to 10D/20D/1D standards but it will not feel bad with a heavy lens hanging from it, which is the most important thing. If you can really afford the 20D/L-lens combo, go for it; if that is too much, you will not regret at all getting the 300D.