Have a look at
www.steves-digicams.com.
I have a Nikon D70, they also make a cheaper D50 which is well rated.
I got a 18-70mm lens with the D70 and bought others off ebay for a third of retail prices. Dont be put off non-Nikon lenses - for motorsport Tokina, Sigma and Tamron are fine in my opinion and much cheaper too. A standard film 100mm lens gives 150mm on a Nikon Digital due to the size of the image chip.
The Nikon D200 has just been released which is pro level - might be a better long term bet because of its higher megapixels and fast overall action.