AF of a Rebel XS vs. 50/60D?

I'm getting ready to upgrade bodies soon and one of my factors is the the accuracy/speed of AF within the new body. From those with experience, how drastic is the difference in AF accuracy and speed between a Rebel and either the 50 or 60D? I know the 7D kicks butt, but I think I'd rather go with either the 50 or 60D and the new Sigma 17-50 f/2.8.
I'm not shooting race cars or Olympic bobsledders, but I'd like better tracking capabilities with my new body. Thanks in advance

Ir's a stark difference especially with faster glass. If you only use the center AF point you might not notice too much difference but the outer AF points accuracy/speed are quite a bit better. Tracking capabilities are better but if you're subject goes out of frame often then try the 7d. My friend showed me how well it can recover focus on an object that falls out of frame and then back into frame. It's great for panning with fast or erratic moving objects. It was quite a step up from my old 50d.

The 40D-60D have 9 cross AF points. This means the outer 8 points are as accurate and fast as the center point on your XS. The center point is also cross type, but adds a high precision X type sensor that assists when using f2.8 or faster glass. So basically, it is a lot better, yes - at every focus point.

This is a little different than your specific situation. But having owned the XS, T1i and T2i and now the 5DII. I can tell you without a doubt the 5DII is far more accurate at AF, though the Rebels were never particularly bad. The 7D and 1DIV would of course be the kings of AF though.