What Castguy2003 said. Go servo all the way!
I can't think of any way that you would keep the gantry square unless you somehow drive it with the screw down the centerline. Even then, off-center loading ( cutter load on extreme edges) could give you jam-ups and inaccuracy. I think two parallel screws would be the simpler answer, and give the machine much needed rigidity.
I was imagining that you could create a truss table, buying a bunch of that fancy T-slotted aluminum extrusion that you can get from Reid Tool Supply. I'm not sure how thick a top you were planning on using, but chances are that the top itself is not going to be all the flat to start with, and if it was, it will sag anyways.
With a truss setup, you can prestress and flatten your table sheet. You would need an upper and a lower parallel bar for each truss, and link the ends together with a vertical straight link. Then you can apply diagonal bracing between these two beams, to straighten and prestress the table. This might allow you to use a lighter top, since top thickness does next to nothing for vertical bracing anyways.
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