RDRAM compatability

Q? How can I get my upgraded RDRAM (pc1066) to boot.
Details: Dell 8250, 2.4 Ghz PC
2×128 modules work great; upgrade-2×256 shut down as windows tries to boot. I have worked with Dell and they think it is a problem with Windows.
I am not an expert but I can assure you that we have tried every config. combination and conclude that factory only provided modules may be the problem if windows is not the problem