Since you onnly mention image quality as a reason for upgrading, and as you are delighted with the nifty fifty, here is some contravertial advice: DON'T UPGRADE.
There are are a few reasons to upgrade the kit 18-55 lens:
-you might want a faster, or constant, or even a faster constant aperture, say if you do portraiture or video.
-you might want better construction, say a filter ring / hood mount that doesn't rorate
- you might want faster focusing, quieter focusing, full time manual over-ride.
These are all fair enough reasons to upgrade, edpecially if you have a particular interest such as landscape where polarisers grads and hoods are used, or sports where speed is of the essence.
But here's the rub: ever since the first 18-55is ( first shipped with the 450d) the image quality has been very good, and these lenses represent excellent value.
You would need to spend quite serious money to get anything even slightly better.
I would suggest using your 'upgrade' money to expand your range, maybe a telezoom or another fast aperture prime lens ( a short trle like the 100mm f2.0?)
If you want a faster aperture then I find the digma 18-50 f2.8 dc macro a very good lens, the newer 17-50 f2,8 os version is reputed to be even better.