around 153 watt idle probably 400-500 watt at full pelt gaming.
About right for the idle, neither the Q6600 nor the 8800GTX are power efficient really, but 400-500W gaming, thats highly unlikely, Gaming rarely loads the cpu and gpu to max at the same time, it kinda alternates (very rapidly) from one to the other.
My system with a Q6700@3Ghz, and a 8800GTX never broke 300W while gaming. Now I've swapped the 8800GTX for an ATi5850 and both load and idle have dropped.
Anyway assuming a balance between gaming, and idle, your probably looking at around 200W for the PC, and another 30-50W for the monitor depending how big the screen is.
On standby the PC willl use about 2watts, in S3 deep sleep it will probably use 10-15watts depending on how much ram you have installed, in hibernate to disk, it will be around 2w the same as full standby.
PS I wouldnt recommend turning the mains power off to save the 2w load, when the PC is getting mains power (but computer is off) the PSU still sends a tiny 5v power to the bios/clock and a few other bits and pieces, and with the mains turned completely off, the bios and clock resort to using the battery to keep running. The battery will last longer with the PC plugged in and the mains on (PC in standby/off is fine).
All depends how much you use the PC.. 24/7/365 and you are talking £260ish based on 12p/unit.