I made the jump from the 450D to a 50D a while back and am very pleased. Don't expect any significant changes in IQ, however the handling and focus system are big improvements over the 450D. The MFA ability on the 50D may or may not be important to you. It's not something I've ever needed, even with primes when I had them. I will contend if the OP is shooting motorsports, the improvement in FPS will be a big win. I was at Rockingham Speedway this spring for a race weekend, and was shooting with just a friends T2i from the infield/pit road, and the decrease in frame rate ability was something I immediately noticed.
Much is made of the noise of the 50D vs others, but as with most bodies when pushing into the upper ISO ranges, it's capable of good results with proper lighting. I found it's best to stick with the native ISO's vs incremental 1/3 step ISO changes.