Hi wat3rpolo, welcome to the Cameralabs forums!
An IS lens will help reduce camera shake on subjects which stay still - so IS will help keep your background sharp, but not your riders. And if you're moving the camera to follow the riders, then IS won't help at all.
What you want is a lens which focuses quickly - which in Canon's world means one with USM. If it also has IS, then great for normal use, but it may not be helping for all your action shots.
You'll also be using fast shutter speeds to freeze the action which means gathering lots of light. So you're also looking for a lens with as small an f-number as possible, and or a big flashgun. The latter are really useful for closerange action shots like these.
Zoom lenses with small f-numbers can be very expensive - check out the Canon EF-S 17-55mm f2.8
But if you can live without a zoom, then consider doing your action shots with a fixed lens like the 50mm f1.4 - relatively affordable, very bright aperture (four times brighter than an f2.8 ), and quick USM focusing. If you were asking about portraits, I'd suggest the 50mm f1.8, but it doesn't have USM focusing, so isn't as good for action. I was at a snowboard event where the pros were shooting with Canon 50mm f1.2 lenses, but that's serious money.
So what's your budget? If you've got some cash, I'd say go for the EOS 40D plus the 580EX II flashgun and either the 50mm f1.4 or if you can afford it, the 17-55mm f2.8. Not cheap, but a lovely package!