I’ve always found gnome desktops to be something of a hassle to find good themes for. Usually it takes a solid amount of time rooting around in one of the repositories, checking compatibility, etc. etc.
One of the places I’ve found that has a reliable theme pack that’ll install and give you a clean looking desktop to work with is the Bisigi Project. Just pop their repositories in your system, install, and go.
Gnome 3 support is coming with the release of Ubuntu 12.04. For now you can hack it in for previous versions of gnome or get it working with LXDE.