Intel vs. Radeon vs. Nouveau On Linux 3.15 + Mesa 10.3-devel
The Radeon HD 6000 series, while old and most Windows gamers are past them, for open-source Linux fans they are currently among the best buying choices for having a high-performance and relatively stable free driver experience without having to use Catalyst for Linux gaming.
The Nouveau performance should be quite interesting once the re-clocking / dynamic power management is sorted out, but there isn't yet a firm time-frame when we can expect this open-source NVIDIA driver to have the support properly implemented. Dealing with GPU re-clocking is one of the most complicated areas of the modern GPU drivers.