Why are cross-country events typically on crap days?
It's been quite a while since competing myself (~15 years), but the premise of getting wet and/or cold still seems to hold true.
As noted by all, pushing the exposure seems to be causing at least some of the softness. Having had the *IST DS and the K200D, there is a lot more shadow detail available to pull from the K200D (same sensor as K10D), Albeit with a bit of noise. So don't be too afraid to underexpose a little. centre-weight the metering.
The contrast will also be poor generally due to the flat light coming down above the rider.
Technique
Shooting horses presents different challenges, since unlike racing (except monster trucks etc), the action is not all on one plane. With horses, there is a lot of side-side, up-down, slow down-speed up!
In terms of focusing on the action, I have had better luck pre- focussing on the spot (and it looks like you have chosen good vantages to make the jumps look colossal - down the hill!). Leave the AF in manual, focus to point above jump (about where the riders head will be for good facials) , turn on catch-in focus, and leave your finger on the button (motor drive) for when the rider appears in frame.
The other option I have used is telephoto, to cover a bit more of the DOF, coupled with being ahead and slightly higher than the jump (if the gallop lanes will allow), you can get some nice rider head/horses ears forward shots as they go over the jump.
Tools
If your copy of PS won't handle DNG's, consider playing with the beta of Lightroom 3 (Free at present!), and see what sorts of differences you can generate by shooting DNG.
No clue about the file sizes, sorry. I do shoot RAW however.
I haven't done this sort of shooting since having digital - it might be good for a change - at least one of the forum members here is local and shoots horse events. Good luck, the tool you have is excellent, as you say, just getting to know it.