Fred, almost any PC with an average run of the mill soundcard will do much lower latency (10ms or less) than any workstation or arranger keyboard. Latency is the time from when you hit the key to when you hear the sound. Some of the things which affect latency include:

Hard Drive speed if you use disk streaming samples (7200 rpm with 8 mb Cache).
Average quality soundcard with ASIO drivers.
Amount & quality or RAM ....if you use alot of samples (disk streaming) you should install 2 GBs.

CPU speed has some impact but it is relatively minor.

Finally, there are other general things which can help and they include tweaking Windows XP for multi media or music applications. You should remove things which are not required for music, e.g., Internet Explorer, etc. The HD should be defragmented, etc.

As I have said many times the proof is in the pudding. The vast majority of serious composers and screen writers use PC or Macs for their live and studio music productions. I have switched from arranger keyboards to the software solution many years ago and have never looked back....quality, reliability and efficiency.

I hope this provides some general guidelines.