Good call! I never think of the two valves in the head. You can't tell from the picture, but the hex part of the valve is connected to the rest by an extremely thin bit of metal - so thin, in fact, that you're just as likely to twist the head off and have a nice smooth hole in your head than remove the valve. Nice. The valve on 30v engines has a Torx head embedded in the valve body instead, much nicer.
So: oil pressure relief valves:
- 16 bar (232 psi) in oil pickup assembly
- 11 bar (160 psi) safety valve, only opens at high RPM and cold engine temp; AFC only
- pressure relief valve, *cough* in the oil pump
- now in the engine; there are the pressure valves Mance mentioned, one on each head. Those control top-end oil pressure.
- there are two oil check valves in the steadying chamber (under the valley pan), but those don't really do much in terms of pressure control; they just keep oil in the lifters when you shut the engine off.
There's also some pressure control done by the head gasket - each one has a finely-tuned obstruction to oil flow that maintains oil pressure in the engine block when the oil pressure relief valves are open.