Interested in visiting Uzbekistan

I am an American citizen interested in visiting Uzbekistan this summer. I have several questions.

1) If I am visiting for just a short period of time (3 days) for the exclusive purpose of visiting a friend, do I need a visa? If so, is a tourist visa sufficient or do I need a private visitor visa?

2) If I go for a longer period of time and cross the land border to visit Afghanistan, what visa do I need?

Thanks!