Two development workstations at my company show the same symptoms.
They are configured as follows:

Windows XP x64 Edition
Athlon X2 3800+
ABIT AN8 (nForce4 Ultra) motherboard
Albatron 6600LE 256MB PCIe video card
1x hp pavilion f1903 LCD monitor on DVI-D
1x Samsung Syncmaster 955DF CRT monitor on VGA
84.21 WHQL ForceWare driver
Running in DualView mode

Often times the display stutters when in accelerated mode, by which I
mean objects will move forward, then backward, then forward again
quickly. It's easily noticeable in the OpenGL screensavers included in
Windows. It also affects transition effects in Picasa.

I suspect it may be a bug in the x64 driver since none of our users
with 32-bit XP have complained about the problem.

Also, if 4x AA + 8x AF is turned on in the video driver, running any of
the built-in OpenGL screen savers will instantly hard-lock the PC. But
disabling one monitor seems to circumvent that problem.

Are there any known issues with dual monitors on dual-core X64 systems?
Do I perhaps have a pair of bad Albatron video cards?