You'd actually see very little improvemnt in image quality, especially if you're not printing larger than maybe 11x14".
The main advantages of the more expensive models are additional features, not image quality. The 7D has all sorts of features. A better AF system, a built in electonic level indicator, microadjustment of AF on a lens by lens basis. Full 1080 HD video, a larger viewfinder, lower noise at high ISO settings and so on. You also get a rear QCD (quick control dial) which is very convenient for quickly changing exposure compensation and a few other things. The 7D also has better sealing against dust and moisture.
If you're just taking normal pictures under normal lighting and making prints up to 8x12 or 11x14, you won't gain anything significant by moving to an EOS 7D.
On the other hand if you're taking pictures in low light, using tracking AF and making 20x30 prints then the 7D will give you better results.
I'd say an upgrade to a 50D wouldn't really be withwhile (the 50D still doesn't have video). For a significant increase in usability and a small increase in image quality you'd really want to upgrade to the
EOS 7D