You can get a web license for ITC Garamond here: http://www.myfonts.com/fonts/itc/garamond/ Disclaimer: MyFonts webfonts are usually shitty auto-conversions of print fonts.

Depending on the context, it might be wise to not let the logo dictate the rest. A quick suggestion for a more versatile alternative: MVB Verdigris. It has a relativly “silent” text variant, and a display variant with higher contrast and taller x-height.

As for a sans serif companion: What roles should the different typefaces fill? Headlines, text? Serif and sans paired at text sizes?

You can inspect more closely what ITC Garamond, a typeface many don’t like a lot, looks like as a webfont at fonts.com. Could be worse actually.

Thanks for the advice. For web fonts, I am more familiar with Typekit, which allows to visualize instantly the render of the font on different browsers/systems. Is there a difference between typekit and fonts.com in term of web conversion?

For the Sans serif companion, here's what I am thinking about:
Display Titles: ITC Garamond
Subtitles: sans-serif
body text: ITC Garamond and I need an alternative for web texts. I currently use Adobe Garamond pro from Typekit.

I will give some trials replacing ITC Garamond for another alternative, MVB Verdigris is effectively pretty close (I also looked at Plantin), but both are more "font for book" rather than "Display fashion-ish fonts", which is what I need. So I might maybe end up using ITC Garamond as the body text/secondary typeface and the sans-serif for Headlines.
Any thoughts about a sans-serif?
Thank you!

Did you see that, when you click on one of the weights, you can get samples for different OS and browsers? It’s the rightmost tab labeled ‘Web Font’.

ah awesome, thank you R, I didn't see it at first.

Glad to help!

One more thing: I have noticed that some fonts available on both fonts.com and Typekit don’t render the same when comparing the live preview of both services. It is unclear to me if this has something to do with the service itself or with the font data they get from the foundries although I suspect that only the latter matters.

Interesting. Typekit has a great reputation, from what I heard.

I also found Genath, which is beautiful and offers Display and body text options. It's obviously more condensed but I really like it. The font is available on http://www.webink.com. Webink don't offer a preview render, they only suggest a minimum size. Have you tried them?

I am still looking for a pretty neutral/geometrical font complement to use at approx 15px., and only uppercase. I found Avenir on MyFonts.com (I rather use Typekit or Webink) and Proxima Nova on Typekit (However I would prefer a font with no contrast at all...). If anyone has a good imput on this I'd be very grateful.