I usually go with a screen optimized font like Ariel in 12 or 14pt.
Bingo! Same here.
I would avoid Times and other serif fonts as they are harder to read onscreen. Serifs do better in print.
Generally speaking, that is true, although when used sparingly, Georgia makes a great font for headings.
I'd add Verdana and Tahoma as nice, simple fonts that are very readable onscreen.
Verdana is a good "fluffing" font for when you want the text to take up more space. It is pretty close to Ariel, but wider. A favorite of high school students when writing reports. I wouldn't use it for a long email or if you tend to be verbose, since the width will make the email seem even longer than it is.
Yes - I like Georgia. I find it slightly more elegant than Times. It's also slightly larger at the same point size, which I suppose is a part of the appeal.
As for Verdana, yeah... It's quite wide. I've never had to deal with those kinds of issues though -- fluffing reports.
I can't stand Chinese or Japanese fonts for the Roman alphabet. They're simply painful to look at. Many Korean fonts are similarly painful, though in general they are readable with some even being pleasant onscreen. I've really never noticed anything odd in other non-English fonts.
I think the safest font around is Arial Unicode MS. It's simply easy to read and complete. Few machines don't have it, which makes it very safe to use.