I've been to Monza a couple of times, and both times had a great experience. Italians have a very different attitude to us english, and so like them don't worry about parking and getting in too early.
The first time we went by train, from the main station at Monza they run free buses to the circuit and back all day long. The last time we drove, and you will be directed to a car park. Then you can walk in or presumably grab a bus. We didn't get there until late morning and were told to park in the central reservation on a main road, they do charge you but it wasn't much and then we walked in, as I say they do things very differently and they are very friendly people. Not sure how long the train would take from Milan but their train service is very good.
The circuit itself is in the middle of an enormous park, so it's a fair walk into and around, but the atmosphere and sound there is fantastic.
The first time we only had general admission tickets and that was fine in itself as you can walk round and see pretty much everything, but the locals all camp along the trackside so it's hard to see anything at some places. It is fantastic to see the locals bringing in their own home made grandstands, and that's not a joke, have a wonder around and you'll see what I mean. The only problem we had was it's hard to follow exactly what happens unless you speak Italian and can understand the commentary, but some Italian lads helped us out with a few hand signals.
Last time (2003) we had seats in the junior grandstand, and for the money they were excellent, and I think all the grandstands have a view of a TV feed which is a big help. I think we parked up around 11 ish and were in our seats by about 12.
I'd leave a little more time than viamichelin's estimate though, they are good but there’s bound to be delays somewhere, depends how fast you drive
Leave Nice as early as you can and enjoy the drive. It's well worth it for a completely different experience.