If you need to change the fonts within your browser, you can just use command+ + or - with FireFox and Safari.
For the finder, you can go View-->View Options and then adjust the font size for your folders/file names.
As far as adjusting the menu bar fonts, not sure. You may be able to use a program called Quartz Debug, but don't take my word for that. I haven't used it before either, so user beware...heh