Assuming you can get about 800 RPM or less from a stepper motor at 48 volts that is 13.33 rev /sec. That means you have to send 2000 * 13.33 pulses per sec = 26,600 PPS. So just about any old P4 running MACH at 35KHZ core frequency would work fine. We recommend a 1.8GHZ or better and 256M RAM. Really basic PC. They go for 80 to 129 bucks on ebay all the time.
The 800 RPM is pretty much full out with a small load. We actually use 650 RPM with the G540 as a design number. It will depend on the motor itself to some extent but that is with modern hybrid steppers. Older steppers won't quite hit that RPM.
Totally Modular CNC Electronics