I just built a CNC router parts 48x48 and I'm very happy with it. You didn't say but what kind of speeds are you looking for. I can traverse at 400ipm and cut up to 150ipm (with the nema 23 package they sell)
If you wanted to improve upon that I would just put Hiwin rails and nema 34's on them and you would have one darn stout machine.
The Xzero machines look nice as well. I may upgrade my K2 z axis sometime to one of their Hiwin Z axis with a ball screw.
Xzero CNC - CNCzone.com-The Largest Machinist Community on the net!