What hardware information about your PC can be transferred ever the internet or stored by websites or by your Internet Service Provider.
For eg. Your MAC address can be stored by websites but I would like to know what else can be stored and known.
Hardware can only leak out if there is a specific plugin for it (not a usual case), or if some activeX checks.
A new HTML5 element
Have a look at http://panopticlick.eff.org/ for an interesting look at what your browser tells a site.
As far as I know, your MAC address isn't exposed to any website - if it was, why would, say, advertisers bother with trying to track you via so many different methods, when there was one foolproof one? None of your PC's hardware specs are included in the request, though your browser will be exposed in its User Agent string, and that often includes OS, too. You can change that trivially, though, so it's not foolproof by any means. Your ISP can't see your mac address either, iirc, as I do not believe that ever leaves your local network, assuming you're connecting via a router.