Multiple Monitors in Windows XP

I have had good results setting this up in W98 and W2000 but not getting anywhere with WXP. Microsoft’s HCL doesn’t specifically list cards approved for this in XP. However, my manufacturer’s booklet for the cards says they’re compatible. Spent an hour on the phone with the SiS Tech, but still didn’t get it to work.
I’ve got two PCI cards and an AGP card – all identical – no combination seems to work.
The cards are installed – Primary works fine, secondary is recognized, then listed as “notworking” in Device Manager. Only approved driver available is native XP driver.

Anybody know of any new tricks, or cards that definitely will work?