visas – Interested in visiting Uzbekistan

I am a US citizen wishing to visit Uzbekistan this summer. I have several questions.

1) If I am visiting for a short period (3 days) for the sole 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) What visa do I need if I leave longer and cross the land border to Afghanistan?

Thank you!