This is an interesting question, I have some thoughts on it.
As big mike said, the difference between, say, a 400d and a 7d is mostly just features and AF performance. I don't use a 1ds2 because I think it'll make me a better photographer, I use it because it has features I like - world class AF, 45 AF points, full frame beauty and dynamic range, etc etc. it's more that,
because I'm not an amateur and I know how to push my equipment, that I look for certain things in a camera - not the other way around, per se.
Also, a prime will typically yield better image "quality" than even an L zoom lense. But, what the L series gives us is very, very nice autofocus performance! The difference is big IMHO. I will usually use a prime if i'm in the studio, but when things are fast and footlose, I'll use an L zoom, for like weddings and events, and babies and children.
Now, when it comes to bodies, I prefer full frame. I'd rather have a used 5d than a 7d for the type of shooting I do. Noise performance is also a factor for me. I can shoot pretty good with the 1ds2 up to at least 800 and even 1000 and things look great especially the parts that are properly exposed (the shadow areas are sometimes a bit noisy, but can be cleaned up and smoothed out in post).
If I were someone starting out, if I had more than enough money, I'd get a used 5d. If I were someone who didn't have much money, I'd get a used 20d. either way, I'd spend most of my time reading books and improving myself as a photographer, and not worry much about the equipment until at least a couple of years.
Another thing is that, LIGHTING equipment means more than anything - the best camera in the world doesn't help if the lighting sucks
