You can do this by specifying your own stylesheet, though it takes a bit of work. In Safari 3, the option is in the Advanced tab of the Preferences dialog. I don't remember where it is in Safari 2. In your stylesheet, you just put a lot of lines that look like this:
"HTML tag you want to change" {font-family: "font you want" !important}
The "!important" thing is important because it forces Safari to override any settings from the website's stylesheet. However, I don't know what happens if a website's style sheet also specifies "!important" for a particular tag. Hmmm...