If you are looking for a progressive way of upgrading, then a "newer" computer would be the thing. I would keep your old computer to run your present system for the time being.
Although Mach will run with Vista, I don't know how it performs, so look through the forum on Vista and see what you think. I use XP and it is fine.
A second hand computer running windows XP (with USB 2.0) would probably be as good, and cheaper. (Dont bother with the dual boot). If I were setting up Mach now, I would use a Smooth Stepper. The advantage is that all the connections between the computer and the lathe/mill come through a USB connection. You can, therefore, change machines in an instant, and if you are looking to control three machines all from the same computer, this would be simpler. Mach can run seperate versions, each tailored to a particular machine, with seperate settings for each machine.
Start with your Myford, then your router, and finally, when these two are working, buy a Smooth Stepper for the old machine, and upgrade that.
Is this a hobby type shop, or professional - I had a Myford, but I have a Warco now - it is great with CNC.