The drives will be analogue velocity signal or at least normally on commercial machines they will be 99.9% of the time.
If the drives are good, then you will need some data and 3 of the Skyco cards to convert step/dir to a velocity command . These seem to have a good reputation, and are cheaper than most alternatives.
As Brian said above you could go the whole hog and put Brushless drives and motors on, but that will cost a fair bit. I see his point, but don't really agree that it's mandatory to making a machine reliable. Brushed systems are equally reliable, but never quite as fast and do every few years require their brushes changed.
400"/min is doable with brushed motors as your original setup proves. Going much faster would be pretty pointless and scary on an open machine like yours, I'm used to 1500"/min machining centres and even they make me twitchy sometimes!
The limiting factor on your speed will be the resolution of your encoders and the availble pulse rate from Mach.
Have you looked to see what encoders you have?
Wayne....