Just read through the last few posts about pulleys and cable's and must admit after using those old fashioned drawing boards a while a go that it does seem like it could work.

But...

Instead of all the pissing about with pulleys why not make each of your your gantry sides a little wider so that you can space your linear bearings further apart to prevent racking, yes you would loose travel distance but it may be easier, this is what I have done with mine for exactly the same reason. I have also upgraded my X rails to THK 20mm and put two bearings on each side, slightly overkill for what I am doing but better safe than sorry.

How accurate do you want your machine to be?

How much do you not want to spend on your X and Y motors?? and what screw are you think of using??

I know how you feel threorizing which is why I went out and bought a load of parts to build my machine this week before I thought about it anymore, whatever you build either wont be right or you wont be happy wih it but you have to start somewhere. If you do go with the pulley and cable idea then I would be interested to see how it works out as I think it has potential.