There's some good info here about mixing tires:
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=136 Bottom line, as long as the other two tires are similar in construction, performance category, and speed rating, you should be okay. Best is still all four tires the same though, and make sure you rotate them so you don't wind up in this situation again.

Oh, and same with winter tires (four is best), but if you have to use just two, they always go on the rear - doesn't matter whether the car is FWD or RWD.