Wait until the final release and don't use the natty alpha if you want accelerated opengl with great performance or else use the open source driver radeon (i use the gallium3d accelerated drivers) that supports 3d on some of the cards
"The binary video drivers -fglrx and -nvidia do not have XServer 1.10 compatibility, so do not function in Alpha 2. We anticipate receiving an updated driver with this support from NVIDIA in the coming weeks, and an updated -fglrx from AMD at some point prior to Natty's release, but do not know their exact ETAs."
Also The unity3d doesn't work in catalyst so stick to lxde with radeon driver
The good news is my dual monitor setup works with the default radeon driver
no crashes , the 3d is something like 60 FPS in glgears (that is quite slow)
Monitor settings in gnome seems that
xrandr --output DVI-0 --right-of VGA-0
ps: one mention is that is better if you cleanup the xorg of monitors (i had dual monitor setup with fglrx) also remove the fglrx crap