Sometimes it helps to get down & dirty with these Optiplex machines. I have a GX620 that's BSOD'ing once a week. The best tools I've used are the Windows Debugging Tools.
http://www.microsoft.com/whdc/devtoo...g/default.mspx
(You'll probably have to download the Symbols package and configure the debugger to look wherever it is you downloaded the symbols to). If you don't ,your output will be all fudged up.

This debugger will tell you what's most likely causing the BSOD. Basically, install this program. Install the symbols. Grab your minidumps from (c:\Windows\Minidump) and look for
Probably caused by :
towards the bottom of your output.

Then pray it doesn't point to ntoskrnl.exe. Like mine does. That's a more general error and requires further investigating.