2gtx 460 should serve u well.it ouperforms gtx 480 at less than 500 dollars.
He said he can't SLI...
Looking at the prices on that site the GTX 470 is your best option considering it basically is the same price as the HD5850. It will however prevent you from having the option of adding another in the future on that motherboard as a upgrade path. If that is important to you then get the HD5850 instead. I would recommend this one, especially if you plan on overclocking; http://www.komplett.dk/k/ki.aspx?sku=598966
It is the best HD5850 you can buy and allows for voltage modification of the core for the best overclock(often up to or over 1ghz.)
Since i am buying a new computer around march 2011 xfire or sli does not matter. As long as i can run most games maxed out on 1920x1080 i am happy.
Then get the GTX 470 and when you upgrade grab an SLI motherboard so you have a nice upgrade path should you want it.
Alternately you could get a GTX 460 for now and overclock it. It would probably be your best choice for the money. It is not quite on the level of the GTX 470 but it is much more power efficient and gives off much less heat. The GTX 460 can also OC a ton(about 35%) and when you do it will do very well for your resolution. Then later maybe get another for SLI when you get a new motherboard. SLI scales very well 2 would be very close to the performance of an HD5970 but cost much less.
@ Great_dane: Whichever gives the best warranty in your country.
Jyjjy makes a valid point, though: You might want to sacrifice a little performance now (and save a little cash) with a single GTX460 and SLI with the future build.
Of course, you COULD aklways SLI the GTX470 .