If it's an old one w/ SiS 5598 chipset aka TXpro-II, then there's a jumper. If it's newer (SiS 530 chipset), then you don't disable it ever. The integrated VGA is multi display capable, and coexists happily with another VGA in a PCI slot. BIOS there lets you choose which one becomes the primary output, that's all.
There still are cards that can't handle this situation though. Bad track record on cheap NVidia chip based stuff, good success rate with ATi. Make sure you can return the card in case it doesn't work.
regards, Peter