I'll spare you the gory details, but if you want hardware-accelerated 3D with a Radeon card and an Intel chipset via the fglrx driver that the Restricted Drivers manager will install for you, add the following two lines to /etc/modprobe.d/blacklist:
On my old Dell 8300, at least, the fglrx driver cannot detect the external AGPGART interface if these modules are loaded.
And then you can't use Beryl, because fglrx doesn't support the Composite extension.
So you need to use the Radeon driver and AIGL, but you need to LD_PRELOAD the fglrx GL library before loading the Beryl manager.
Not big. Not clever.
Also, Beryl is a bit rubbish - with the exception of the whizzy desktop cube thing. The rest of it is a bit pointless.