I don't believe it's a problem of wrong placed ram sticks, this would just blank your screen and halt the power on self test with some beeps

It's possible that you need to place ram in pairs. Like 2x128 and 2x64. Do you have the manual of your motherboard?

No? The best thing you can do is look at your motherboard, read out the model number and make, and search google for it using that number (or directly go to the manufacturers website, changes are you will find the manual for your board in 2 seconds. In that manual you will see the compatibility. Some boards do not accept certain memory sticks.

Are you sure all memory sticks are at same speed, type, ... ? (like said above)