You'd be best advised to shop around for a better price on RDRAM.
In order to get the same bandwidth from DDR as you have with RDRAM on your "400" bus processor, you'd have to use either PC3200 in single channel mode (no chipsets support this option with your CPU to my knowlege), or go Dual Channel with a very GOOD dual channel board!
Plus you'd have to replace your case!
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<font color=red>Only a place as big as the internet could be home to an ego as large as Crashman's!</font color=red>