I actually have right now two questions:
1) What font faces are preferred for a website? Right now I really like \'Segoe UI\', but it\'s only available on newest Win
Use the right typeface for your site. It's not as simple as saying "this is the best font for websites". Here are two quotes (read principles) from Robert Bringhurst's "The Elements of Typographic Style":
On another note, unless a serif face really suits the website, sans-serif faces are more appropriate for digital media.
With regards to your second point, Phil Oye has recently released FontUnstack, a jQuery plugin which will tell you what font is being used for a given element. The element will be given a class (i.e. "set_in_gill_sans") through which you can apply specific styles for different typefaces.
Also, check out the font matrix (1.5 years old) and these well thought-out font-stacks.