cecilc
TPF Noob!
There isn't much difference between the 30D and 20D, so you might as well go with the 20D or 40D, but as said before, you can get much better glass with the 20D.
Sorry, but there is a significant difference between the 20D and the 30D ....
On the 20D (and earlier 10D) you can only adjust ISO in full stops. Starting with the 30D, you can adjust ISO in 1/3 stop increments. That's huge .... that's the difference between nailing the exposure or only getting it "close" .... (my opinion only, folks!) ....
But I will go along with the opinion that lenses are a bigger and longer term investment than the camera body. Fast lenses are never out of style and are always the best in low-light situations.