Flight Sim doesn't stress your GPU that much so no brainer here. Unless you're going to play Crysis in 5760x1200px you shall be more than happy with triple monitor setup
tbh i wasnt happy with 5850 performance in COD4 or Far Cry 2 either on single screen (not tried 3 with 5850) so i would def go 5870 for flight sim multi monitor setup.