Spent some time with SketchUp this evening, and some of the ideas here, and here's what I've personally about decided to go with for an assembly table:
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I'm feeling cheap, so the top will probably be a cut down 30" x 6' 8" hollow core door, which I already have in storage, and with a laminate on the surface. The rest is just tubafours.
Castors should put at just the right height for my purposes, it's narrow enough to move around the existing workbench and out the door, and a 3" overhang all around allows a clamping hold. I like the 30" width because you can easily use cauls and the table edge to clamp across a work piece at that width, and you can always throw a wider top on temporarily if necessary.