The best setup is all dependant on exactly what sort of motorsport you are wanting to do and how far away from the action you are.
I'd say your current kit should be fine, certainly for rallying where you are generally quite close to the action and even for track based motorsport dependant on the circuit (not Silverstone as the distances and wire fences are not tog friendly). I've got some shots I'm happy with at Oulton Park with my D80 + 70-300mm (no where near the 300mm end), the aperture isn't a problem if you shoot side on and pan, the blurred background comes from that (I'll try and dig up an example - but may have to wait as I think the photos are at home and I'm away on business this week).
To behonest to really capture the movement and excitement it's the skill of the photographer that will make the difference so my advice would be go with the kit you have now and master the craft first before spending vast quantities of cash on more lenses. Although if you do want to buy more I'd suggest going for f2.8's as these will be better in low light situations and to help control Depth of Field, they are also generally better quality glass. 80-200mm f2.8's are generally available S/H at reasonable prices if you can't afford or wait to save for the gold standard 70- 200mm f2.8 VR at £1.3K.
