Lion for multiple computers

Can I buy Lion once and install it on my computer as well as another computer in the household, or do I need to buy a copy for each computer?

Also - will dropbox work just as well in Lion as it did in SL? I heard that this might be an issue, but not sure if it was resolved or not.

Lastly - My MBP (bought 2 yrs ago) is definitely slower/more laggy than it was when I bought it.. will Lion slow it down even further? Or maybe speed it up?

P.S. The only reason I am considering upgrading is to get Reminders (iOS) to sync to my computer... if anyone knows another way to do this, please let me know