If it is a SLI capable motherboard, you can put another card in it.
Bear in mind said card needs to be identical, right down to firmware, manufacturer/brand, die size, etc. Your PSU would need to be up to snuff as well.
For a 9800, SLI would be a waste IMHO. Could go out and buy a cheap 600 series card and have 10x the performance of 2 9800's. Even 2 9800's would not do well at higher resolutions and settings with those games you mentioned, they are 4+ year old graphics cards.