Encoder count selection for G320

Hello all,
I want to use homeshopCNC servo motors on a router. The motor is rated
72VDC and 4200 rpm no load speed, 850 in oz peak torque, 170 oz in
continuous torque. My plan for the gantry drive is to go from the
motor through a 1:3 reduction and drive a jackshaft with 14 tooth XL
gears on each end, which rack over stationary belts fixed to the
frame. I calculated the gantry mass at 175lbs aprox. I'd like to
achieve 30 ipm traverse speeds for the gantry and carriage. Is that an
unrealistic speed?
I don't know how to make the choice between 250 or 500 line quadrature
encoders. The 'files' sections servo motor files were empty. From what
I remember, top speed depends on pulse throughput from the PC. The
control software is CNCPRO, Turbocnc, and EMC. Would the 500 line
encoder provide greater precision?
Would anyone care to show me how to make the line count selection for
the Gecko G320 and advise on the PC speed/performance requirements?
TIA
Denis