What's a good replacement font for ITC Avant Garde Std?
Century Gothic is pretty well-saturated on the Web and has letterforms that are in the same ballpark as Avant Garde. If a Mac doesn't have it on their system, you can put Futura lower down the font stack.
Not sure how scientific these surveys are, but the numbers are in line with what I've read before: Century Gothic is on probably about 87% of PCs (63% of Macs too!) and Futura is on about 96% of Macs.
Tex Gyre Adventor is a really nice replacement for both Avant Garde and Century Gothic. Useful if you also target devices that are not a pc or Mac. It's available as a @font-face kit at FontSquirrel: http://www.fontsquirrel.com/fonts/TeX-Gyre-Adventor