On the assumption that both forces would be numerically even (for talks sake) I think that a campaign by Alexander into Italy would have been a very hard fought event. For one thing Alexanders troops wouldn't have come up against anyone like the Romans before. Even in their infancy the Romans infantry were more than a match for a phalanx of pikemen. The reason i believe that they were a match for Alexander is that the Romans are famous for thier adaptability and i feel that they may have adapted to Alexander's use of phalanx and cavalry. Also although the Romans themselves only really had some light cavalry, some of the cities in Italy (especially in the south) had heavy cavalry. The Romans later emloyed them into their army. Alexander was supposed to be brilliant at exploiting the enemys weakness. However he'd never encountered anything like the Romans before so he wouldn't know their weakness. However in the end I'd say the Macedonians victory would be enevitable due to thier experience in battle. The only way I could see Rome really winning is if they could dicide on the field of battle and picked a terrain unsuitable for large cavalry movements. Maybe a marshy terrain.