Hmmm. You need a lens that will go from fairly wide-angle (to photograph Porsches up close, and get the whole car into the frame) to real telephoto (to photograph Porsches on the track, rather far from you, and have the car fill the frame). You need a camera with minimal shutter lag (the delay between pressing the shutter button and the camera setting exposure, autofocusing, and then taking the picture). You also need a camera that will take several shots in quick succession. You need a camera that will take some video. Finally, you don't want to spend too much.
Given the budget, I think you should look at the Panasonic Lumix FZ20.
It has an excellent lens, a Leica zoom that covers 36mm to 436mm (in 35mm film camera equivalent). 36mm is a moderate wide-angle, 436mm is a long telephoto that will reach out and fill the frame with a race car 100 ft away. The lens is fast (f2.8 throughout the range), which isn't so crucial for you, since it sounds like you'll be shooting in daylight, but does allow faster shutter speeds (to freeze action) and wide apertures (to get selective focus). The lens can be manually focused using a traditional focus ring (useful at the track, if you want to prefocus on a corner and shoot as cars come through). The lens has image stabilization, useful when hand-holding at 436mm. It will shoot a burst at 2 or 3 frames/second. It takes short movie clips in Quicktime. The full shutter lag is only so-so at 1 second, but if you have the shutter button half-pressed (i.e. the camera autofocus has already focused), then the lag from there is very short (1/10 sec). I'm going to guess that in manual focus mode, the full shutter lag would be very short.
The negatives of the FZ20 are that it uses SD cards, not CF cards, si it can't use the 1+GB microdrive CF cards (tiny hard drives). With the falling price of flash memory, this may be less of an issue. It also has "noise" in the image when using film speeds (there's no film, of course, so this really means the light sensitivity setting) above 200 ISO. For daylight shooting and with the fast f2.8 lens, I think 200 ISO is fine.
The FZ20 is around $600, I think. The older FZ10 might still be available, its pretty similar.
The Panasonic FZ5 (replaced the FZ3) is quite similar, but is a slightly smaller camera and lacks the manual focus feature, and the hotshoe for an exernal flash. It apparently has a bit less full shutter lag and may have a bit less noise at 400 ISO. I think it is somewhat cheaper.
Myself, I would get the FZ20 for the manual focus option.
I wouldn't buy a digital SLR (like the Canon Digital Rebel) unless you plan to ultimately spend $2000+ on the body and a couple of lenses.
I would not buy a compact point-and-shoot type digital camera. They won't get the pictures you're describing.
Personally, my SLRs are film since I like to develop and print the old-fashioned way. But I've used the FZ20 and talked to users, and that is the digital camera I'd get if I were going to spend <$1000.
Go to
www.dpreview.com for more info and detailed tests.