I will give you my $0.02, since you asked. Take it with a grain of salt.
Does anyone have anything good or bad to say about using 2 pieces of 2" square tubing with4 inches in between them for the acme screw in the center
Good= Can be done cheaply.
Bad= Can be unstable.
I guess it all boils down to how much you want to invest, and what kind of tolerances your trying to achieve. My router
http://sinistersam.com/mill/router.html is made the "cheap" way. 2X2 extruded aluminum tubing with a span of around 50". Kind of like the one in the picture. I have found out that without proper support, you will get a bit of flex when cutting forces are applied. Not a sideways or vertical flex, but rather a front/back tilt flex. Leaning back in a recliner comes to mind. Its not very much, but its enough to not build another one that way. If your using linear rod
without a center support, I would guess you will get some flex anyhow. Linear ways
are a better choice for a multitude of reasons, but are costly. A solid gantry like rceebuilder's
http://www.machsupport.com/forum/index.php/topic,6766.0.html would be my idea of a good solid gantry. Werther or not you need one that rigid, is up to you. Even with the amount of flex I have with a 50 inch gantry, I would not hesitate to tackle any woodworking project that comes to mind. Engraving detailed patterns in aluminum is really where it rears its ugly demon head. Good luck!