Troubles with one Dell laptop, two video cards, and three monitors

I've been struggling for several months now to set up more than two simultaneous displays on my Latitude D830, running Windows XP. The laptop has an internal Nvidia Quadro NVS 140M with support for 2 displays max; I want to run the laptop display, plus two Dell 19" widescreens. Recently, on advice from a message board (http://forums.nvidia.com/lofiversion/index.php?t28778.html) I bought an Nvidia Quadro NVS 280 PCI card and a D/Dock docking station. My idea was to run the laptop display and one monitor off the internal 140M while running the second monitor off the 280, installed in the D/Dock's PCI expansion slot.

I installed the latest drivers for both cards: the Dell drivers for my internal 140M, and the latest official drivers for the 280 (version 169.47). Driver installation seemed to go OK, Windows detected the card and all three displays, and all three showed up in the NVidia control panel. However, the display hooked up to the NVS 280 was badly garbled and essentially nonfunctional. I could move the cursor around, but dragging folder windows over resulted in brief system freezes and display corruption once the windows were moved back to a different screen. Even the basic Nvidia diagnostic screens (color test, etc.) wouldn't show up on the third display. Worse, my system became badly unstable, and Windows froze several times. I had to use system restore once, after which I attempted to reinstall the drivers for both cards and try again. The symptoms didn't change, and eventually Windows crashed hard and announced that my display drivers had "stopped working." At this point I undocked the laptop and came to MG...

I'm really at my wit's end here and I have very few ideas as to what the problem might be. I've read that troubles might arise if you're not using the same drivers for both the docking station card and the laptop's internal card, but then the 140M and 280 require different drivers. Someone in that thread I linked above reports successfully using an almost identical setup (his internal card was a Quadro 110M, mine is a 140M; I'm using XP rather than Vista; the external card and docking station are the same). I've also read that power to the D/Dock might be an issue, but it 'feels' more like a driver or OS problem. I'd be extremely grateful if anyone has any suggestions that might bring me towards a solution, even if it means buying a different PCI card for the D/Dock (but one known to work), etc.

Thanks.