I don't understand (g)mch and (SPD). Help me out please

I don't understand (g)mch and (SPD) how they work together in the bios. I have a ep45t-ud3p running at q6600.

I'm trying to clock but when I have all theses like A,B,C,D and then all the dividers that come with it.

so I can run my FSB up to 400 that will give me ram speed of 1600. but then I can run a combo of things.

I am lost.

all (g) mch does is enable u to use a particular multiplier.
Set it on auto and you can try them all.
I found B worked best for me .D worked ok from 390-460fsb,after that I found better results with B again.

I just for that too. but even with the other letters I can have some of the same multipliers.

some of them give me the same speeds on the ram. so whats the difference?

I've been wondering the same thing too. I just upgraded to the UD3R and was poking around.
Couldn't figure out the letter differences though