I was thinking about buying a new screen, and it would be nice to get some more screen-estate, so I'm tempted to get a 27".

I'm currently running an AMD Radeon 4850 using the Mesa R700 drivers, which work well on a Gnome 3 desktop, running at a resolution of 1680x1050. To my knowledge (based on old Phoronix articles), the 2D desktop performance of the FOSS drivers is even better than the Catalyst performance, but now that most of the desktop is "3D" (OpenGL accelerated) I'm not sure what the status is. I initially bought an ATI/AMD card to support their FOSS initiative, so I'm reluctant to change to the Catalyst driver.

So what's your experience: Is the Mesa R600/700 driver performance good enough for a 2560x1440 resolution on a Gnome 3 desktop?

I would think about buying a new graphics card if it was necessary, but since the performance of the driver is best for R600/700 (also according to Phoronix performance testing), it just feels like a waste of money.