I would like a sound system build for my Home PC that can reach up to 120dB.

Military jet aircraft take-off from aircraft carrier with afterburner at 50 ft (130 dB)

So you seriously want to put a Military jet in your living room to blow out every window in your house?
 
I have seen some stereo systems in cars that hit like 160 with subwoofers and thats what im going for it just didnt seem like 120 would be as loud as you guys are saying. I don't understand how the decibel system works all to well.
 
The sound level meter in that video was not a calibrated one and may have been off by as much as 15-20db in the bass region so you can't use that as a guide to how loud that system actually plays.
Max volume is less important than quality of sound. Boomy loud bass doesn't give you the same sound that Steven Spielberg wants you to get. Being able to hear foley effects that are low in the mix and clear dialogue are more important. Get enough volume for your room and then go for better sound not more.