Also was stuck at 1600 with Lion 10.7.4, MB: asus P8H67 revision 3 (B3), CPU: Core i5 2400, GPU: GTS450, RAM: 8MB.
So far everything works fine: sound, graphics, sleep, speedstep etc.
Resolved with:
1. flashing bios with official P8H67-ASUS-3603.ROM
2. bios settings:
Ai overclock tuner...