Hi Ian
The extra memory wouldn't (shouldn't) have changed anything in the driver tests. It is purely a test of the ability of the paralell port to put out pulses.
2gb of RAM is the maximum that XP can handle and is really not required for Mach3. 500 meg is more than sufficient for a computer dedicated to running your CNC.
A better thing to do would be to optimise the computer for Mach3 as per their instructions
here.
Also, try a couple of speeds that are a little more than you require. Sometimes it makes things go a little more smoothly. (Note the requirement to restart the computer if you change the kernel speed settings.)