400MHz memory on Dell 4500

I'm trying to install a 400MHz 512meg memory on my Dell 4500, that
should use 266MHz memories. I think that a faster memory should work
fine on a slower computer, but the only thing I get are 3 beeps at
power up that means ram problem. I also try to boot the computer with
the original 256Meg ram module on connector 0 and the new on connector
1, but to no avail.
The bios of dell 4500 is very limited and do not have setting for the
memory.
I will appreciate very much any help.

Giovanni

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