I would go with the x2 but I just don't have the money to buy that on top of a new system. I was planning on getting either the hd 4870 or gtx 260 and then getting another in a few months when I've got some more cash. Of course, if I do end up having enough money, I might just go with the x2.
That is exactly what I did when I built my PC. I purchased one HD4870 1GB and than couple of months later I got myself a second hd4870 1GB.
It's a great combo and it works flawlessly.