Nina,
I take it that you don't find the fps of the 50D vs the 7D to be a major issue with action/sports photography. I'm lusting after the 7D but really could get the 50D and be very happy as long as it works as far as action/movement shots. Of course, I've been working with the 40D and it's done just fine in those situations. Like you, I have not seen a huge improvement in high-ISO shots between the 7D and the 50D but both are much better than the 40D. It's time to move up but with a house move and medical expenses, it has to make economic sense.

Jim