Welcome to the club, Nuke!!
Yes, I have the 20D. The Canon 300mm F/4.0L IS I highly recommend for Airshows.
That kit lens you got you'll retire soon. It's not that good.
Highly recommended(depending on what you shoot):
70-200mm F/2.8L IS
24-70mm F/2.8L
85mm F/1.8
50mm F/1.4
17-40mm F/4.0L
70-200mm F/4.0L
400mm F/5.6L
I'm looking to get Canon's new 10-22mm F/3.5-4.5 USM for my super wide angle needs. Just waiting for the reviews/test results.
For compact flash, get Sandisk's Ultra II series. Best performance for the buck (2GB for about $185.00). Shoot in RAW as that will give you best image results. I really recommend you get Photoshop CS.
I recommend you get the battery grip as well. You can shoot all day without changing your batteries.
If doing a lot of indoor shooting, invest in an external flash unit.
Heck, this is probably making your head spin. I'll stop now.