Well, I will explain my problem.

I have two computers, computer A and computer B. I also have a memory kit (2x1Gb) which we will call dim X and dim Y.

If I mount X and Y in computer A, life is beautiful, all works properly.

If I mount X in computer A, life is beautiful, all works properly.

If I mount Y in computer A, life is beautiful, all works properly.

So computer A works fine.

If I mount dim X on computer B, life is beautiful, all works properly.

If I mount X and Y in computer B, system doesn´t start because my motherboard says that there is a memory error.

If I mount dim Y on computer B, system doesn´t start because my motherboard says that there is a memory error.

So, the module Y doesn´t work on computer B.

Incompatibility maybe? Not possible, because dim Y is exactly equal as X and works fine.

Faulty module maybe? Not possible, because dim X works fine in computer A (tested with Memtest).

So, where is the problem? It is too much weird

Computer A: Asus P4V800D-X
Computer B: Asus A8N-SLI Premium
Memory kit: 2x1Gb Crucial Ballistix PC4000

What is the motherboard in PC B? Is it reading the timings right?

CPU? PSU?

Motherboard in PC B is an Asus A8N-SLI Premium. I tryied auto timings and memory specs timings without success. I also tryied several bios

PC B:

Asus A8N-SLI Premium
Zalman ZM600-HP
AMD Opteron 180

Computer B works fine with 2x1024 Corsair XMS Platinum DDR400 CL2

After looking at the motherboard manual I have noticed that the dimm slots are in channel format and this will be noticable on the motherboard. dimm A1 & A2 B1 & B2
if yo intend to install multiple ram sticks they have to be maching pairs
Instalation is important you must install them in this configuration A1 + B1 = 1 pair A2 + B2 = the second pair Page 2-11 in the user manual will explain this.
As said before the you should see two blue dimm slots and two black dimm slots this denotes the slots that are paired up.

Thanks for respone. I tried it without success. I finally contacted the memory manufacturer and tramited RMA. I sent the memory modules, so I am waiting for the new ones