Please take no offense to my suggestion.
N4 is demanding on video cards and XP is demanding on memory. I'll share my
setup that yeilds 78 fps in 1124X960 32 bit color;
Asus 7n8x running the 400 Mhz FSB
AMD XP3200
ATI 9800 Pro w Cat 3.5 drivers
21" Sony E540
Invotek Wating Cooling
ECCI Pocono Controls
XP setting tweaked via Tweak XP software
My XP has Service Patch 1 installed and highly tweak by Tweak XP software.
I don't use most of the dymanic shading in N4
I play around with game graphic settings until reached compromise between
visual and frame rates.
I have heard that XP service patch caused some of the problems you described
and there is a patch for that at the Window Update center. Your Browser
should have the update link under tools.
Force feedback devices can't make things choppy.
Sound card setting can slow things down too. I believe mine is set at 13.
Your best area of improvment would be the video card.
I hope this helps,
Rush
Manor Texas