well you got my attention there..... lol
ok to start off:
1. I don't think it really is at 1600MHZ
2. some people said that installing a 64-bit OS will bump it up, although when you think about it, the BIOS doesn't know what kind of OS you have until you are booting from it.
according to this
site your motherboard supports a maximum FSB of 300MHZ
the only socket 754 board I've seen that apparently supports the whole 1600MHZ is the Gigabyte K8N Pro, when most boards are only 800MHZ; although if your mobo really does have a maximum of 1200MHZ, you might be able to 'overclock' it in the BIOS settings, otherwise my only answer would be: your motherboard doesn't actually support it