Autofocus is definitely better on a 1 series than 40D.
1D 4Mpx, 8fps, 1.3 crop
1D II 8Mpx, 8.5fps, 1.3 crop
1D IIN 8Mpx, 8.5fps, 1.3 crop - same sensor and focusing as 1DII but bigger screen and slightly different interface
1Ds 11Mpx, full frame
1Ds II 16Mpx, full frame, 4fps
I used a 1DII and reckon its great. They are significantly cheaper than the "N" version and take the same pictures at the same speed. For sports and action, I reckon they are a good bet.
They are slightly better at high ISO than the 40D from my experience but you will lose a bit of crop factor - which may, or may not, be important to you.