so um, you replaced your 4850 with a 9800gtx?
.... lol. cards are very very similar in performance.

and you want to add another? i'm assuming you have at least 2 pcie slots because you have an i7, meaning x58 chipset

you can add another 9800gtx and run 4 monitors yes, each video card will have 2 monitors
buuuut, i'm sure your mobo won't support sli because gateway is cheap like that and didnt buy the licensing for it
so you won't ever be able to setup the cards in actual SLI
you won't be able to get 4 monitors out of either one of these cards alone (4850 or 9800gtx) but if you havent changed out your video card yet, go with another 4850. if you have, go for the 9800gtx.
you can't really mix-match, windows doesnt like 2 driver sets