You should upgrade your lenses first, they will give you a much better overall quality (and pleasure!) than a new 7D while not having the right lenses. Also, I am seeing this quite a lot, but the newer sensors of the 7D, 60D, 550D > etc. actually have better sensors than the cameras before them, but really, the differences are not as huge as you may think. I own the 50D and 350D, and while the 50D isnt as good as the newest bodies (differences are quite small in real life though), the 350D and 450D (XSi) are not the same either. Maybe it helps to say that I happily shoot at 1600 ISO with the 350D, and that the 50D's 3200 ISO isnt that much different.
To keep it short: the 450D/XSi is an excellent camera. Keep it and buy some better glass first. Maybe youll fall in love again with the 450D, which will save you some bucks!
Panasonic DMC-FZ18, Panasonic DMC-FZ28, Canon G5, Canon 350D, Canon 50D + BG-E2N
Tamron 17-50 2.8, Canon EF 70-200mm f/4L USM,
Canon 18-55 II plus lots of Minolta MD/M42 lenses and bodies