Introduction and MTB shooting advice

Hello everyone. Thanks to Photo-John for making me aware of this site via the MTBR forums.

I just jumped to still photography from video in June with the purchase of a Canon 30D. I've always been wanting to get great action shots as well as good artsy shots, but would get quickly frustrated with P&S cameras that I have owned in the past. The main problem with P&S was the shutter lag which is probably why I was more into video. So since i've gotten my camera i've been doing lots of reading about photography and am also taking a photography class at the local JC. While i'm comfortable being behind a lens in general due to my video experience, still photography is a tough but welcome learning experience.

What i'd like to ask is if anyone has any advice, tricks, tips and techniques for shooting Mountian bike photography with the equipment I currently have or equipment that may be worth the investment. One thing that I find challenging is the low light but high shutter speed requirements of MTB in Nor Cal because often times it's under tree cover.

My equipment selections currently includes:

Canon 30D
70-200 f4L
50 1.8
18-55 kit
.42x semi fish that can be attched to my kit lens
220EZ flash

What i'd like to add in the near future:

Good replacement for the kit lens. Maybe the 17-40 F4L just don't know if it will be fast enough.
Battery grip
Sigma 8mm fisheye.

Here are a couple of the better shots i've come out with but lots of them don't come out. Again any critique advice would be greatly appreciated.