ram slots 3 and 4 don't work

I don't know why, but the system configuration says 1333mhz will only work in 1,2 or in 1,2,3,4

so does that mean they all have to be 1333mhz to use all 4? I think so.

but the are all 1333MHz 

but how can i look at the bios if my pc dont start up with the new ram.... and yes with the normaal 8gb i can see in the bios that it find all 8gb

Then I'm uncertain either.. As long as you have a 64bit O/S it should work. I don't see any restriction on mixing sizes as long as they are paired. I'm assuming your looking at BIOS first to make sure they see the total amount available.

here is how to tell if you have a  32 bit or 64 bit system. most likely you have a 32 bit if you have a 64 bit then something may be wrong with the ram slots. 

Instructions

1. Windows Vista and Windows 7

   • 1

     Click the "Start" button, type "System" in the search box and press "Enter."

   • 2

     Click the "System Summary" option in the left navigation pane.

   • 3

     Find "System Type" in the right informational window and make note of your system's edition.

   Windows XP

   • 4

     Click the "Start" button and choose "Run."

   • 5

     Type in "sysdm.cpl" in the search box.

   • 6

     Click the "General" tab. If Windows lists "X64 Edition," it is a 64-bit version of Windows, if it does not list an edition, it is a X86 32-bit edition.

i know how i see if it is a 32 or 64 bit system and it is a 64bit. can it by that i need to activat slot 3 and 4 with somthing like a jumper or in the bios?

I have a few things you can try

You mentioned you already had 8GB before you purchased the 2 x 2GB's. Is the old RAM 2 x 4GB 1333MHz (So you have 2 x 2GB and 2 x 4GB is that right)?

If you have 2 x 2GB and 2 x 4GB try alternating them between the slots ie Slot 1 = 4GB, Slot 2 = 2GB, Slot 3 = 4GB, Slot 4 = 2GB.

What CPU do you have a i5 or i7?

Do you have windows 7 home premium or something else?

Have you tried swapping the RAM around to see if its a faulty RAM stick or a slot?

Can you remember what BIOS version you are running? Check if theres an upgrade and try that, use you old RAM to do it.

Did you change anything in the system BIOS in the overclocking menu?

i already alternat and it does nothing at all its a i7 860 2.8ghz and windows premium 7 64bit and i swap the ram and i think that its the slots that dont work i have the last bios version and i didnt change anything in the bios. and i tought its a alienware lets buy that it can by bad but my first "easy" upgrade the ram doesn't work lol

Might be time for a new mobo then, R2 mobo are really cheap now, heres a link to one on ebay

http://www.ebay.com.au/itm/Dell-Alienware-Aurora-R2-motherboard-0RV30W-P55-/220809036349?pt=Motherboards&hash=item33693e823d

or you could go for an R4, the only thing you'll have to change beside the mobo is the top IO panel where the USB connections are, the R2 and R4 have different connectors from there to the mobo

http://www.ebay.com.au/itm/Alienware-Aurora-R4-Motherboard-/200987800896?pt=Motherboards&hash=item2ecbce7540

If your in Australia I think I may have a R2 mobo floating around somewhere!!!

have you tried the new ram on its own in the known working slots? .. it might be a conflict between  OEM ram and the aftermarket you got.. IE different timings/voltages ect..