What do you plan on cutting with your machine? IMO, your long X-axis rail support "beams" should be given additional support. Perhaps run another "beam" parallel to the main one and tie them together with uprights every 6" to 10". I realize you are using "Supported Rails", however if you plan on routing hardwoods or aluminium (even with shallow DOC), you will need additional stiffness/rigidity along that X-axis (especialy if your gantry has any weight to it). As it is early in your build it shouldn't be too difficult to add it now, as oppossed to trying to add it in the later stages.
So, what do you plan on cutting/making with your machine? Please give us all the vital statistics (working envelope, motor sizes, electronics package, router/spindle, etc.).
Good luck with your build. Thanks for posting, axiously awaiting additional updates with pics!