Dell T5400- I love the case and setup

I bought an old Workstation which needed some work. It runs Dual Quad Core Xeons (2.66) with 20 gigs of RAM, a reasonable NVIDIA Video Card, a 240 gig Revodrive and a secondary 1 TB 7200rpm secondary. I like the setup. All of the PCI card upgrades are appropriate and it is a good case with good air movement and incredibly quiet. Running Ubuntu 15.04 desktop (i368). I use it for business operations, word processing, media player and security system.

I know that I am a major noob but I like to see what kinda frankenstein monster I can build. I am simply looking for a more modern MoBo that will fit and work in the T5400 tower.

Ideas? Budget is only limited by how much I can sneak past my wife.
 
What does i368 mean? I really hope that isn't some typo and you are running 32 bit code. If you are, you aren't using more than a small portion (approx 4 gigs) of your RAM.
 
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