So it sounds as if Eric covered all the bases. Get as close as possible, try to shoot as someone is coming AT you, vs. left to right and it will be much easier to catch the rider in the middle of the frame.
When I shoot at the track, I start at 400 shutter and as I get warmed up and tracking better, I step down until I can't keep the rider in frame anymore. Many of the top shooters can track, frame and nail the shot at 125.
I shot at Iowa Speedway this wekend and 200 provided plenty of crisp and still had spoke blur.
Tracking is all about time and practice, use a monopod to make it ONLY left right or right left as the tracking requires. There are TONS of close shot opportunities at RA, so no excuses there about not being close enough to shoot 200mm.
DO NOT use the teleconverter unless you have to. It cuts the light available in half (doubling you f-stop). If there is enough light, you might experiment with it, but it needs to be VERY bright out.
Finally, experiment with manual focus. Set the distance to the apex in the viewframe at a crisp focus, and then shoot in that spot everytime as you track the bike. No waiting for the focus to engage.
F-stop - The lower the number the shorter the focus are is. SO... I shoot at 2.8 as often as possible so that the target is crisp, and everything else is blurred. harder to shoot, but makes for nice shots.
ISO - the lower the number the less grainy the image, especially when you zoom in or blow it up. Keep the number as low as you can and still accomplish your other goals. I would guess D200 starts to shred images at 1200-1600, I wold look at http://www.dpreview.com
for the exact numbers. Keep it at 400 or lower if you can. if you have GREAT lighting, shoot at 20 or even 100.
Finally, these are all really basic questions. Send your wife to Wolf Camera for a camera class on speed, aperture and ISO.