Ive been a couple of times now, but the other half has relatives who live in Tangmere, and we walked there from their house
Anyway, get a program and make sure you can be somewhere near the track for the cars you want to see (for us it was the F1 and the supercars), at the right times.
After this I wandered round the manufacturers bit (the Giulietta was just launching), then we headed over to the concourse d'elegance to the side of the house. After the F1 cars we headed up to the rally section in the forsest, and the rally paddock at the top, where there were plenty of old Italian classics, from Statos(es?), Fiat 131, Delta Integrales etc.. Be warned though its quite a hike to the top...
I found I could get most of what you want in a day, and I was (content) and knackered when we got back to Tangmere!
There is also a "bar" area with live music if you want to rest your weary feet!