When I designed my machine I started with the moving table idea but after a few hours on the CAD software decided that it would be easier to make the machine more rigid by using a moving gantry and good quality linear slides, I went for 20mm THK which are fully supported, easy to mount and do a very good job.

Take a look at my build post if you have not already - [ame="http://www.mycncuk.com/forums/showthread.php?t=1058"]My Build - I blame you lot for this....! - MYCNCUK[/ame]

The sizes that you are talking about are very similar to mine, I have a machining area of 800mm x 500mm. The sizes for my machine are in a PDF download on the first post.

My design is quite simple and was easy to put together, I just went a bit over the top with material sizes to get the rigidity and it worked!