[jogger08152]

Couple thoughts:

I know you didn't ask, but for whatever it's worth, ruling out hostels is a bummer, imo. I spent ~6 mos in Europe, part of the time flying solo and part with two different groups of friends, during and just after college. Spent maybe 10 days or 2 weeks in Italy, staying in both hostels and hotels; I'm 33 now and would repeat in a heartbeat either way, but both the frequency and the content of hostel conversation is vastly better. (You'll probably do this anyway, but make sure to snag a copy of Let's Go, Berlitz, etc. If you enjoy touring cathedrals/museums/etc, these will give you some good suggestions and make the trip both more efficient and more fun.)

I think your trip would be slightly more fun with a friend (or friend with benefits, especially in venice... but somebody you're comfortable with would be better than somebody you're trying to impress (too easy to miss the cool local stuff if you're distracted)) but if you like to travel, are into history, art, Italian culture and/or cuisine, I'd expect you to have a fine time either alone or with company.

If you make it to Florence, spend a day at the Uffizi even if you're not that into museums. It's amazing.