I'd go with 20mm rails and 1610 ballscrews on both X and Y.
discounted automation products: BLH Linear Guideway, BLH 20mm Linear Guideways
And 15 or 20mm rails for the Z axis.
discounted automation products: BLH Linear Guideway, BLH 15mm Linear Guideways
Take a look at this thread for some ideas:
http://www.cnczone.com/forums/diy-cn...28752-cnc.html