Currently the Opteron 246 systems are the record-setters for the SW benchmark. The comparison I saw did not include Xeon processors though. Anyway, I don't think Dell or Compaq sell anything but Intel. The Xeon porcessor is not significantly faster than the P4 on the SW benchmarks. See
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Solidworks does not scale at all to two processors. A second processor won't be needed unless your users need to do analysis or rendering in a separate application in the background, or do other things while waiting for SW. Photoworks does use both processors. See if your people use it.
Make sure the RAM is as fast as possible of course. Stay away from Rambus' high latency. Look for dual channel DDR DRAM on the 800 MHz bus. In my opinion, SW and Windows are not stable unough to warrant ECC RAM, and it's speed penalties. Others may disagree. If the system does not use the registered DIMMs, make sure the memory is installed in two and only two modules for minimum latency. For more, see the discussion of memory here:
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Check the task manager's memory usage when your users have a system under what they would consider maximum load. Get more memory than that, obviously. No rule of thumb has proven effective, since memory usage varies so widely with application, even in the same industry. I seem to be using 15-20% more memory since moving to Sw 2004. You may want to take that into accound if you haven't moved from 2003 yet.
The QuadroFX500 video card should do well. Some might suggest a fast GeforceFX with a SoftQuadro patch on the drivers to save money. I'm guessing you don't want to run unsupported drivers.
If you have room on the desks, a quality 21" CRT will provide better resolution, brightness, etc. for a lot less money.
Money saved on processors and monitors would be best spent on making sure the RAM is optimal and a fast disk subsystem. Asseblies and drawings for a normal project can run into hundreds of MB on disk. Obviously, the faster that dat acan be read into memory, the sooner you people can get to work.
Almost anything is going to be lightning fast compared to your existing machines. Your users must get a lot of web surfing done while waiting on those things.