Linux PCB with support for GPU accelerated X Windows
I'd suggest the MK802, or some otherboard like that built around the Allwinner A10, but AFAIK, the Mali 400 used on the board is still not supported by the Linux distros available for it, just the built-in Android...

There is also the Mele A2000, a media box that has more features, it is a complete computer featuring even SATA - and the Allwinner A10.

I recentlz found a more hackable board, that exposes some GPIO pins too, but I can't find it now...

Distros on MK802