Re: So why should I buy a GTX280 again?
I have owned both 4870 Crossfire and 280 GTX SLi. Neither of them provided a noticeable benefit over a single 280 GTX for me, unless I stared at a framerate counter. The games that Crossfire and SLi work well with a single 280 GTX already gets insanely high framerates, it is impossible to tell the difference between 100 fps and 200 fps. And I find that SLi and Crossfire don't always help that much in terms of minimum framerate.
With Crossfire and SLi you end up having to wait for patches, wait for new drivers etc etc. 280 GTX gives you consistent performance and no headaches.