Geforce 9800 GT

[Duce]

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.

[Duce]

That is not true as long as its the same gpu and memroy size it does not matter what brand it is just as long as its the same card. I ran 2 8800gt's in SLI one card was zotac and the other BFG and worked brilliantly.

Earlier models were more forgiving with it, as are pure reference designs.
Most people don't buy the simple reference design models unless they are extremely early adopters, though.  I have seen different brans work occasionally together, and does only really apply to 100% reference models, but it'd be foolish to tell this guy it WILL work.  I've got two old 9800's here of different brand type, as well as 2 285's of different manufacturer that I simply know do *not* work because they are different brands.  I've tried them, and they don't work in the least in SLI form together.

nVidia themseves say to only use the same manufacturer, so I'd rather not confuse people, heh.

Crossfire is a whole different animal altogether.