I am a student at the moment, so I am on a fairly tight budget. I want to use this computer for various different types of applications, such as messing around with virtualization and file sharing over a LAN and domain creation and other "needs" as they arise. I could definitely see the plus to gaining 4 extra cores with the dual x5650 setup, especially in creating VMs, but they cost about 5 to 10x as much on ebay. I got myself a little giddy at the thought of spending $5 on a working Xeon e5000 series chip, so spending $50 per chip for the x5620 seems nearly absurd now. Of course, if my needs outgrow the e5620's, then I can pony up and buy the x5620's when the time comes, and I will have lost only $10.
And for the primary OS, I plan on using Fedora. That idea may bomb, though, because LInux can be a B when it comes to software compatibility and usability.
I probably will buy a case with a PSU, like the t7500, so thanks for telling me about it.