pny adp648tm-50 memory not working with redhat linux

I had two high speed 1gb mem installed on my primary mem slots. When i tried to add two PNY adp648tm-50 mem, 1gb each on the two empty mem slots, it didn't work.

However both the old and the pny mem by themselves work and the system boot up, but fails if I put all the memories in there.

Any help or suggestions would be greatly appreciated.

Thank you,