Could someone please explain what following 3 settings mean in the BIOS of my P5K SE:

transaction booster ;
cpu gtl voltage reference ;
clock over-charging voltage.

I've set them all on auto, because i dont have a clue what these mean.
Ive OC my e8500 to 4Ghz, so maybe these settings can help me to get a higher OC.

To be frank I'd stop there otherwise you run the risk of burning out the chip.

To be honist, I have never touched any of those settings. All you really need to wory about is your north bridge voltage, CPU voltage, FSB frequency and RAM settings. If you already have 4Ghz I would not wory about trying for more. There is nothing you are going to be doing that really needs that much power.