The design you have linked to is a good start. For the X-rails (4-5' ?) you should put some more supports in the middle to avoid having a long unsupported section

The spreadsheet Mocha referred to can be found by searching 'gantry stiffness calculation':
http://www.mycncuk.com/forums/linear...alculator.html

However it doesn't support extrusion, unless you draw the extrusion in a CAD program (e.g. Solid Edge, Solid Works, Pro-engineer) which will tell you the area moment of inertia for the cross section, which can then be entered into the spreadsheet. You can also sometimes find the numbers on the manufacturer's website.