For whatever reason, Sun systems running at much lower clock speeds did large apps better than Windows (from my observations). That's changed with faster x86 processors though.
I haven't work on any of those, but speaking of debugging on older systems with limited debuggers vs. Visual Studio, there is a big difference in favor of Windows based systems.
Slapping a few Opterons with 16 or 32 gigabytes of memory is going to get you some really interested lookers as the entire concept of disk drives goes out the window.
Databases and file serving is where a lot of server money is spent on, and a lots of RAM is just what the doctor ordered for these systems. The 4 way HP can take 64GB without any expansion. Microsoft has been talking about further substantial gains from 64 bit OS.
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