Gryshnak--
I was in your exact position back in March. Last summer I shot 50 rolls of film at only a few airshows and decided to stop wasting my money on processing and get a new camera...
So I got myself a shiny new RebelXT (350D). I've never been happier! I got a cheap Sigma 100-300mmF4-6.3 lens off ebay just for something with some reach and just to practice with. (note I said cheap... I paid $50 for it but didn't have that good of quality--which I expected) It did served its purpoise and then I moved on to something with longer reach.
The shortest focal lenght I'd go for is 300. Anything higher is better, but usually also more expensive.
I recently purchased the Sigma 50-500mm F/4-6.3 about 4 weeks ago and used it at my first airshow while using DSLR and got GREAT results! This lens set me back $1,200 including two different filters.
And just for bragging rights, I shot as low as 1/30 second and got a couple of really SHARP shots at the 500mm end. That is 100% proof that ANYTHING is possible!! haha And I also have probably a dozen sharp shots at 1/60 sec. And no, it wasn't on a tripod, and not even on my monopod, completely hand-held shooting a gorgeous Boeing Stearman bi-plane at dusk!
And with the lens being slow... if you're shooting props you should never use anything slower than 1/200 anyway. Jets are another story, but unless you're shootoing after the sun has set, you'll always be able to get over 1/200.
Here is one pic I got from last weekend shot at 1/200 all the way in at the 500mm end:

The absolute BEST resource I found during my research was
www.dpreview.com Check out the forums there. This place also has some great lessons and is where I learned all the photography lingo that I know.
Also if you're planing on shooting aircraft mostly, go over to the
www.fencecheck.com forums. That place has some of the most well-know professional AVIATION photographers around the world.
Umm... a lot also depends upon what youre inteding to do with the lens. My goal is to start selling prints right away and upgrade my equipment over the next couple of years.
If you have any other questions feel free to PM me. :) Good luck with your search.