There's nothing wrong with the entry level SLRs like 50/40/450/1000D but your AF will not be as fast. I would have thought the 450d would be a fair performer as it uses the digic 4 chip, having said that, the 40D is far superior in every regard and is very comparable to the 50d. Realistically, when will you ever use the high ISO to get decent pics? The 40D can expand it's ISO to 3200, personally I never go over ISO400, even ISO200 is high for me.
Careful which cameras you group together as "entry level" - 40D/50D is hardly entry level.
BTW, the AF system is not part of the Digic processor - it's a separate system. In the 40D/50D, it's far superior to (in speed, accuracy and sensitivity) to anything found in any of Canon's cameras other than the 1D series. The 5D has an inferior AF system, and from what I've read, it's unchanged in the 5D mkII - time will tell.
For interest's sake, I took my 40D with the 24-105L f/4 to my company xmas party the other night. I have disabled the strobe-AF-assist, because it annoys me, causes pictures with closed eyes, and confuse other people whom I hand the camera to. The venue was much much darker than I expected, so dark, that at times I could not make out faces through the viewfinder, only the silhouettes against the backdrop of a better-lit bar in the background.
Recipe for disaster? Hardly. Out of 53 pictures I took, only one was out of focus. I had the camera in M, shutter set fairly slow (to catch some of the background) and f/4 to f/5.6 mostly, pop-up flash enabled. Of course, on the short end, DOF helps, but I'm referring to the AF points hitting the faces, despite relying on the (nearly non-existent) contrast in the scene.
EDIT: On the 40D you can set it to display the AF points that were active when each shot was taken, so it's easy to see what it focussed on.