It seems that the 9800 GX2 would work better for more high end games than the 280 so if u have one dont upgrade its more of a downgrade and if u have a 9800GTX or GTX+ just SLI it it will probably be cheaper that getting a 280 and it will have a higher fps. However, as i am not a pro please don't base ur judgement of this. It's my opinion
If you are the super hardcore gamer u might wanna check out nvidia's new 3d glasses and monitor for 600. I heard it's pretty good.
i just sold my 9800gtx+ because i wanted to upgrade to the xfx gtx260black
its true the cost might not be worth the increase...but theyre practically giving these things away when you look at the retail bundles....
but anyway, if you plan on staying single card setup then keep your 9800 for now. If you plan on going sli in the future, then i say buy a 260/280 now, and another one later to break the cost apart. do not sli your 9800 it is not worth it ~$300+.