Well, since you already have one 9800GT getting another one to run in SLI would be a cost effective option IF
you already had an SLI board. For an AMD CPU there are only a few good choices for an SLI board since many of them don't have BIOS updates to allow them to accept AM3 CPUs for whatever reason. There is only one board that uses the nVidia 980a and that's the
ASUS M4N82 Deluxehttp://www.newegg.com/Product/Product.aspx?Item=N82E16813131383
It's an AM2+ board so you can use your current CPU for now and it will accept AM3 CPUs. It's designed to be an AM3 chipset but no board maker has decided to man an AM3 board out of it. Anyway if you're looking at a new phenom system I say go with the 4870. The 4850X2 won't do you too much good now since alot of the benefit will be taken up by the CPU overhead of crossfire and thus the 4870 would probably do better with your CPU.