What give you more bang for your buck, better glass or better body.
I mean there is a huge difference in price between hobby body's and pro bodies.
What more do you get (other than a full frame) from a better body than would you get from upgrading from a $300 lens to a $1500 lens?
In simplistic terms:
A better quality lens on your current body will result in a better quality image on your current body (compared to low-end or kit lenses).
A higher end crop body (like a 7D) won't take a better picture than the 550D using a low-end or kit lens.
A higher end full frame body won't even be able to use EF-S kit lenses, but would not see a benefit without an investment in good lenses anyway.
An upgrade to a 5D or 1D body would clearly be ideal, but the cost of such an endeavor would be significant. And even going used, you would need new lenses anyway.
So, because it comes down to "you really need to have new lenses anyway" and "you'll see instant results in better shots with a better lens" it's generally reccomended that you upgrade your lenses first.
The differences in body prices usually boil down to
features: speed, processing, ISO and AF performance, etc. Features that help make shooting in certain situations a lot easier/doable. Of course bigger sensors also add costs as well.
Here's what the ~$600 EF 70-300 IS lens looks against the $2,500 EF 70-200 2.8 IS II when shot on the same camera (I shot this little test shortly after recieving my new lens in the mail a couple months ago):
Long story short: A new lens will see instant improvement in image quality on your current camera and (depending on what kind of lens) would serve as a long-term investment you could use with other high end bodies if you ever chose to upgrade later.