50D would be an excellent choice. AF system is the same in both 40D & 50D, and much better for sports/action than your 5DC's AF. 15MP (vs 10MP in the 40D). It has Microadjust (40D doesn't). 50D has usable ISO 1600, and with some work 3200 (IMO, same as 40D, but at much higher resolution). 40D's highest avail. ISO is 3200... 50D's is two stops higher: 12800. 50D also has improved Live View compared to 40D, as well as a brighter/clearer LCD screen to use with it. 50D supports UDMA memory cards (CF), while 40D doesn't.
Both 40D and 50D use the same BG-E2 or E2N vertical/battery grips and BP-511/511A type batteries & chargers. They also share focus screens and WFT-E3/E3A modules.
50D wasn't a huge upgrade over the 40D and both are quite capable of doing sports/action photography. But particularly at today's used camera prices, I'd be more inclined to go with 50D for some of the modest upgrades it offers.
7D is a more pro-oriented camera with substantially more sophisticated 19-point AF system (with it's own discrete chip, 1D-series style.... In fact I suspect the 7D's AF system is a scaled down version of the 45-point system used in EOS-3 and most 1-series cameras), 100% viewfinder, active/transmissive LCD focus screen (simliar 5DIII and 1DX), dual Digic 4 processors to support 8 fps shooting speed (compared to single Digic 4 in 50D or single Digic III in 40D, supporting just over 6 fps shooting in both models), 63 zone iFCL metering system (a modest upgrade from the 35 zone system on 40D & 50D), and more. Resolution increase from 50D is modest (18MP vs 15MP), but IMO 7D is good for about one stop higher ISO thanks to slightly better noise handling. 50D images appear sharper out of camera than 7D's. This is due to stronger anti-alias filter on 7D and is easily offset with increased sharpening in post processing. 40D's images appear even sharper still because it can use an even weaker AA filter, thanks to the larger photo sites and less crowded sensor. But images from 50D are much more enlargeable or croppable. 40D and 50D both have 100,000 click shutter rating, while 7D has 150,000 click rating. 7D is noticeably heavier than 50D or 40D... probably due to it's 100% viewfinder, plus beefier construction. In fact, it's heavier than 5DC or 5DII (5DIII is only a few grams heavier).