Just thought I'd point out - the bottleneck in all servers listed is definitely going to be I/O. IDE is useless. Go SCSI if you want to take full advantage of the processing power.
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you need to rethink all those power hungry Xeon Irwindale/Dempsey! the AMP requirement is going to kill!
replace dual irwindale with single core2 duo servers, then replace dual Dempsey dual-core with dual woodcrest dual-core. you are saving at least 1 amp per server (60amp worth of saving!), and getting better performance at the same time!
you must think performance per amp for this type of large scale deployment.