Every ATI Radeon I have tried seems to work for dual screen - though I have not tried many of the new top of the line ones. Currently, I am using a Radeon 9800 XT which is a nice solution for those who need AGP, and should be findable on eBay and elsewhere very cheap.
If you just need two displays without super high capabilities, there are also some older Matrox cards out there (but I believe you need the native Matrox drivers to use 2 screens).
I've not been able to get some nVidia cards working (with latest SNAP)... so, if a particular version is supported by SNAP, don't presume that an older one will work for dual head. For instance, if the 6600 is supported, dont presume that it means the 6000 is fully supported.
Also, certain nVidia cards seem to only work for me in "BIOS mode" or whatever it is, and are excruciatingly slow - and many that I have that do work (in either mode) I cant get the second display to work.
Rob