Strange graphic lagginess

I've been experiencing some strange GrafX lagginess when I dual-box on my PC. Basically, even though my 2nd client is capped at 20fps and my main client reports it's running 40+ fps, the picture is still a bit (not too bad, still entirely playable) choppy when my toons are moving.

The strange thing...if I have them on autorun and I pop open my start menu so both wow windows go to the background, POOF the choppiness goes away and it's smooth as butter. Why would it be MORE choppy with my primary wow client active but no choppiness at all when both clients are in the background?

System specs:
Intel Core2 Duo E7400 @ 2.8GHz
4GB single-channel DDR2-800 RAM (lame m/b won't do dual-channel /cry )
WinXP Pro 32-bit (so I'm limited to 3.5GB effective RAM..sides my M/B won't go past 4)
XFX GeForce 9600GSO PCI-Ex16 768MB Video
Both WoW's in their own separate folders, on separate HD's too
one WOW runs windowed/maximized on primary LCD @ 1280x1024 all max details
slave WOW runs windowed/regular on second LCD @ 800x600 all min details, sound/music off

Any ideas short of an expensive hardware upgrade? It seems to me like it's some kind of buggy driver or nVidia hardware issue, especially given that when my primary wow window loses focus the performance cranks up to being smooth as butter (but obviously I can't play with neither wow window having focus).

I'm always leery of upgrading my video drivers unless I know for sure it'll solve my problem(s), I've often in the past upgraded video drivers to attempt to solve an issue and ended up with worse problems with the new drivers...

What happens if you move both windows to one monitor? I was experiencing choppiness with my 5 box setup [1x Character on main monitor, 4x Characters on second monitor]. I moved all my windows to my main monitor using Keyclone or InnerSpace's window layout and my frames were magically better. I read [no technical source to give you right now] that Win XP 32Bit has limitations on dual monitor setups and that 64bit OS's do not. So because I did not like everything jammed on one monitor I decided to upgrade my OS to Vista 64 bit and everything was perfectly fine. I have since then gotten the new Windows 7 RC1 64bit and it's running great.

Here is a post from the DB forum that maybe perhaps will help you out [last post].
Win XP, multiple vid cards, and 2 different monitors