Splitting screens with Win 98

Hi
I want to split a Windows 98 screen with two monitors.
The computer is a Pentium iii and has a RIVA TNT2 32mb AGP screen card installed. I know that I have to put in a second display card, and ordered another Riva TNT2 32mb PCI display card. MyQuestion is, does any one know if I could use a cheaper card like a sis 4mb, or do I have to stick to compatibility as I was told?
Has any one split an AGP and PCI screen cards on an Piii?

Thanks
Melt