You could get anything from an ooold PCI SiS (or is it S3?) VGA card to something a tad bit better like a GF 4 MX .
In most cases it would be better replacing your primary graphics card to one that supports dual-displays (and almost all modern-day graphics cards are capable of it). What card do you have presently?
Also what motherboard and what VGA slot do you have - PCI Express or AGP ?