I'm also planning on having a fair bit of Z travel, as I want to be able to machine moulds from foam. I've opted to raise the height of the X rails well above the table, so the gantry doesn't have to be so tall, and then fit the Y rails with a wide horizontal spacing. At the moment I'm hoping to be able to run the Z axis down between the two Y rails (see other thread on my small diameter DIY spindle to see how this might be possible).

The idea for this configuration came from looking at this machine: http://www.mechmate.com/, although I'm not building something this big (mine's about 800mm x 600mm x 250mm working volume).

Jeremy