Upgrading RAM shuts off comp...plz help!!

Hello,

I am building my first computer build. So far, so good?

Anyway, my computer will boot up with 4gb of ram, but when I put 6 or 8gb it shuts off.

My setup:
Rosewill 750w Power supply.
Gigabyte Mobo
PNY 9800 gtx
Coarsair DDR2 Ram 4 total @ 2gb each


I am waiting for my Hardrive to come in the mail, but I've been able to access BIOS with 4gb of ram and no hardrive.

When I put the other two sticks in, it shuts off. When I take them out, it loads up no problem. Some people say its the power supply, but I just got a high end 750w Rosewill PSU and I'm sure that its not the problem.

Please help, I've been teaching myself everything and this is the first question I've asked for help with. Will share my knowledge in the future =)

Thanks and happy holidays!