DOUBTS WITH STEPPERS MOTORS VELOCITY/PERFOMANCE

HI ANYONE!!I hoked up two "leadshine" microstep drivers, pretty
nice, up to 25600 steps/rev. up to 5.6Amp.
running on a celeron 300mhz, 64mb ram, booting straight DOS, then
runing TCNC,
my configuration of motor is :
max speed 4500Hz
accel 1000hz
slow jog 2500hz
fast 4500hz

when i try to give more speed, the motor speed ups and stalls.
if i try more acceleration, the motor stalls too.

a few days ago , i couldnt run this motors with more than 2500hz,
but reading some post on this group, i realize it could be my Pc. so
i reboot,go to bios setup, disable USB support, and everything i
didnt need...and then i could get my steppers to go up to 4500hz,
ALMOST DOUBLE THE SPEED! so now im wondering,....

whats the max speed should one expect with a stepper motor? (in HZ)
THIS IS MY FIRST EXPERIENCE ; also is good to mention that my
microsteping controllers are setted at 3600 steps/rev. my steppers
works REAL SMOOTH . PRETTY NICE!

i have to try if i decrease the steps/rev to like 1800steps/rev ,or
even 900 steps/rev will i get more processor power at my PC,
(because i will need less steps to make my steppers run )
and then be able to get more HZ at the motor runing....

about my stepper motor, they are MAE really old (about 1986 )
double shaft, 190oz/in, NEMA 34. dont know if they are exactly that
specs, someone in this group pointed me at a site page, with a
similar looking stepper motor made by MAE, so i guess it should be
similar specs too (real dumb deduction too)
idont know the working voltage, but im using them at 48V.

ANY IDEAS??????
THANKS

MARCOS