I just installed Ubuntu 12.10 on a new PC with an Nvidia GTX 560 graphics card, but after installing the Nvidia proprietary drivers (either -current or -current-updates), Unity won't start. When trying to start it manually I get the message "extension GLX missing".

I've searched around and found results like this question which point out it's a problem with Nvidia Optimus laptops. However, I don't have this problem on a laptop, but on a desktop PC.

lshw output for the graphics card:

 *-display
                description: VGA compatible controller
                product: GF114 [GeForce GTX 560 SE]
                vendor: NVIDIA Corporation
                physical id: 0
                bus info: pci@0000:01:00.0
                version: a1
                width: 64 bits
                clock: 33MHz
                capabilities: pm msi pciexpress vga_controller bus_master cap_list rom
                configuration: driver=nouveau latency=0
                resources: irq:16 memory:f4000000-f5ffffff memory:e0000000-e7ffffff memory:e8000000-ebffffff ioport:e000(size=128) memory:f6000000-f607ffff

and CPU:

 *-cpu
          description: CPU
          product: Intel(R) Core(TM) i5-3570K CPU @ 3.40GHz
          vendor: Intel Corp.
          physical id: 40
          bus info: cpu@0
          version: Intel(R) Core(TM) i5-3570K CPU @ 3.40GHz
          slot: SOCKET 0
          size: 1600MHz
          capacity: 3800MHz
          width: 64 bits
          clock: 100MHz
          capabilities: x86-64 fpu fpu_exception wp vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx rdtscp constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm pcid sse4_1 sse4_2 popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm ida arat epb xsaveopt pln pts dtherm tpr_shadow vnmi flexpriority ept vpid fsgsbase smep erms cpufreq
          configuration: cores=4 enabledcores=4 threads=4