Bad news... I tried a 2.8ghz 533FSB Xeon tonight and received the following message at POST:
A 533 MHz system bus processor is installed. This system bus speed is not supported on this system board. The system will run at a reduced processor clock speed and system performance will be impacted.
Do you wish to continue to bott at 400 MHz system bus speed and reduced processor speed? (Y/N)
Press [Y] to continue or press [N] to shut down.
I guess the Dell BIOS programmers took the extra step to make sure we couldn't use this "undocumented feature". This is really disappointing considering that the "new" 600SC with 533mhz FSB support still has the same ServerWorks GC-SL chipset core logic as the 1600SC and the old 600SC. Here's hoping for a BIOS update to make this happen, but I wouldn't hold your breath. I don't think Dell would want their cheap server giving their high-end workstation a run for the money.
The thing that really bums me the most, is that at 400mhz system bus, the DDR only runs at 200mhz instead of 266mhz. I was hoping for better RAM performance from this machine by upgrading to a 533FSB processor. I wonder why Dell even bothers to require PC266, if the RAM only runs at 200MHz?
The 2.8GHz CPU ran fine at 2100Mhz by the way. XP even identified it as a 2.8GHz Xeon running at 2.09GHz.
Just in case anyone is curious, the answer is no, the 2100 (2.8ghz) and 2400 CPUs wouldn't run dual together, not that I expected them to. It didn't even POST.