First,

try to identify the true speed of your system. The speed shown in the BIOS (POST) screen is very often incorrect, my own system shows up at 256 MHz and if that were true, than it would be the world's fastest 256 MHz system (hehe). You are running some kind of OEM mainboard with a BIOS made by AMI (American Megatrends) which shows you the NVRAM wich is supposed to be the display cache but has no impact here since, most likely, you are running the system with the standard SDRAM only (just a hunch)

Do the following before you try anything else:
go to http://www.h-oda.com and download CPUID which will tell you the true speed of your system as well as the entire CPU information, make, model, stepping etc.

If the CPU still shows up incorrect there (it should come up as either 1000 MHz or 750 MHz, depending on the bus speed you are running) then there is something seriously wrong. Most likely, though, you'll see one of these two values.

If it shows at 750 MHz, set the FSB to 133 which should get you to the 1GHz. If it doesn't work, your mainboard is either dirt cheap quality or too old for the new CPUs and someone was glad to find you ...