Ram Problem

Hello,

I have a Gateway DX4200-09 that I'm having problems installing ram in.
Basically the problem is when I place any ram modules into the DIMM3 or DIMM4 slots it won't boot up.
The fans just spin and I get no video. I have what came with it 2gb pc2-6400 in both DIMM1 & DIMM2 slots.
Specs say it is upgradeable to 8gb (2gb per DIMM)
Can someone tell me what might be going on and how I can get 8gb to work in this computer? :confused:

Please Help!

Thank You!

Robqx4