I have read the "how to overclock" guide, but its not specific enough. In the bios, the options to change cpu FSB are greyed out so they can't be selected. I got an Asus P5GD2 Premium 915P
How is the guide not specific enough because your board doesn't allow you to change the FSB? This makes no sense to me.
Check your motherboard for a jumper...this could be why it's greyed out. A jumper could be just set on 'auto' so that it adjusts it's speed according to the CPU.
Im looking through the manual for that motherboard and they kind of suck at addressing the BIOS fully, but it basically says you need to go into the Advanced menu tab and set the AI Overclocking option to 'Manual' and it should allow you to set your own parameters. Same goes for DRAM. Under performance just leave that at Auto or Standard since Turbo is running it at accelerated speeds, but since you will be doing that manually leaving it at auto or standard will offer more stability