Regarding XFire... I really don't know the performance of two Radeon HD 5850 XFired. I think it would be faster than a Radeon HD 7870, but slower than a Radeon HD 7950.
I prefer a single graphic card solution 'cause benchmarks are easier to find, less complications like poor XFire (or SLI) scaling, or no XFire support at all, and a single powerful current generation card will likely use less power than two older powerful cards.
As long as you can live with a mix of medium and high settings @ 1920x1080 resolution, I think the Radeon D 5850 can last you for another 2 or 3 years. As for an upgrade I would say something of at least the power of the Radeon HD 7870, but at a decent price. That likely means a used Radeon card whether it is the HD 7xxx, HD 8xxx or HD 9xxx if you are looking to upgrade sometime in 2014.