Hi All, long time lurker, first time poster.

I have been getting into the photography hobby over the last year. I snagged a bargain at a closing down sale and grabbed a Canon 1100D with the 18-55. Since then I have really gotten into the photography and upgraded to a second hand 50D. At the moment, I am mainly taking photos of mountain bike races. I have a 17-85 which stays on the camera and a 50 f/1.8 which only gets used in low light. The beauty of the sport is that you can stand as close or as far back as you want to get your photos and recently I have found myself getting quite close to the track with focal lengths averaging around 24mm

Most of the time I am also using a Nissin Di622 speetlite to add extra light to the scene are make the riders stand out from the background. The limiting factor though is the max of 1/250 shutter speed which occasionaly results in a blurry photo.
My current plan is to trade the Nissin for a Canon 430EXII, which has the high-speed sync that I am looking for. Is it going to be worth the money for that feature alone?
Also I was thinking of getting hold of a 70-200mm f/4 USM because there are a lot of times where I am hitting the end of the 85mm and wishing i had that extra reach. My only concern is that 70mm might be a bit long for a 50D. I have heard nothing but great things from this lens and it seems many people swear by their 70-200. Do you think this is a wise choice for the kind of work I am doing or am I going to find it too long on a crop body? How far away would I have to be to get, say, 3/4 of a rider in the photo? Would this be too far away for the flash to be effective?

Thanks for any advice.