Think of it this way--a Windows computer has about the same resolution as a fax at it's lowest resolution. You
know how grainy and hard-to-read that can be. Most Macs are set to a screen resolution that only has 40% fewer pixels per inch. You've had people send you faxes in Adobe Garamond--a lovely typeface but it can be almost impossible to read in a fax--and on-screen.
Some people claim that 300 dpi screens are just around the corner. Even if this was true (and it isn't), most people won't have high-res screens for a very long time.
So be kind, and set your type legibly for them.
As I've done here, give people easy links to download typefaces designed especially for the screen, such as Matthew Carter's wonderful Georgia, available for free from Microsoft.This face is so much easier to read on-screen than what they're probably using now that
they may just read more.