If you are wanting more I/O you could always go down the Modbus route, I have purchased a Cubloc system like this one

http://www.henriksplace.se/cnc/cnc_mach3_modbus_1.html but I have not had time to impliment it yet.

The units are now available here in the UK http://www.audon.co.uk/cb405.html#expansion

Going this route might give you more scope for future upgrades to your system.