I have two keyboards, mice and monitors and a ATI Radeon HD 4290 onboard a ASUS M4A89GTD PRO/USB3 mainboard. Can you recommend a distribution or livecd that supports the GPU and offers easy means to setup a dual-head multiseat system where both sessions have direct rendering enabled? (32 or 64 doesn't matter here, although 64 bit are preferred)
I don't have the hardware for test something like this, but I would recommend Gentoo if you like to compile your stuff and be a little late with the latest software.
However, I am using Sabayon right now, it is based on Gentoo (testing repo). They take it, test it, compile it and release it. So, it's a kind of Gentoo on steroids and it is much more user friendly. It should be able to handle what you need.