You will likely have a better experience with the 7850 then with dual 550 ti's for the reasons geofelt mentioned.
However, I will suggest that 'because I have some money' is a poor reason to upgrade your card. If it can run everything you want it to right now, there is no reason to upgrade. Save the money, and then in 6 months or a year when a new game comes out that you decide you need a better card for, the 7850 will be that much cheaper, or there will be a new, more powerful, card for you to buy at the same price.