If you are wanting more performance now, then get the second HD 6850 and the mobo. You can also sell your current HD 6850 and get a more powerful card. You can get another one later on for even more graphics punch. The newer HD 7XXX/Nvidia 6XX GPUs use less energy and would be lighter on your PSU.
I would just get the MOBO now and just use your 6850 untill you can aford the 7xxx series or the 6xx series of nVidia. Crossfiring now with those two cards you will get micro stutter.
The MOBO to get should be able to upgrade the CPU for fucture proof. So really look around for a nice MOBO. But of course you will have to upgrade your PSU. The one now is not recommended.
I mean if you corssfire or SLI with the 7970 and up and the GTX 670 and up. The 600w is not recommende for corssfiring or SLI'ing for those high end cards. But the 7950 and 7870 it would be ok.
I don't know about the GTX 660 Ti. Probably the same as the 7950 and 7850. So use your money for the MOBO save for the better cards and PSU if you use the higher end cards. This is my opinion. Good luck to you.
Some motherboards feature both Crossfire and SLI compatibility, getting one of those now will help future proof whatever decision you decide to make.
Something to keep in mind is that you will probably get better performance (or at least similar) from a Radeon HD 7950 as 2 Radeon HD 6850's and dual Radeon HD 7950's will kick large quantities of ass. Same thing goes for a GeForce GTX 660TI or GeForce GTX 670.