Problems whit RAM

Ok so I was down to my local computer shop this morning.
And I both 2x 2GB DDR2 PC-6400 anyways I get home and put them in my computer. And just like that it didn't want to start.

So I went down to the shop again whit the RAM I already got and the 2 other sticks. They where more ore less identical I changed the RAM to the same type that I already got same specs and so 4th.

So no I shude have 7GB when I use the last stick whit 1 GB of RAM.
It DON'T work it works fine whit 1 of the new and the 2 old ones 5GB total.

When I put in the last new stick the computer starts up flashes the Bios Image Logo ore the Boot Screen for a nano second then it shuts off.
Just so I mentioned everything the 4 slots on my motherboard got 2 different colors. 2 light grayish color and 2 whit black color in the order of Gray, Black, - Gray, Black

Can anyone help me whit this problem?

thanks

 

What make is the motherboard ?
I would look up the maximum ram that the motherboard will take, and if all channels are designed to run in the same ram mode (I have had motherboards that will use 2 sticks of say DDR Ram, or, using the other slots , it would run two sticks of DDR2 Ram-, but not at the same time )

 

another bossibility is the limiting amount of ram channels in some mobos if all slots are occupied the last one or 2 must be single sided ram so if all your ram is the same then your mobo may not be able to run 4 of them.