Upgrade for FSX

Hello Fliers....... I am in the process of upgrading my PC for better FSX flying. My present system is : OS Vista 64bit, PS 750W Corsair HX, MB 790i Ultra core 2 Quad, Processor Core 2 Quad Q9450 2.66GHz O/C'ed to 3.6 air cool, Memory 4GB DDR3 Corsair @ 1600MHz, Video Card Gtx 250 1GB...... My system came with a gtx 280 but it died after 18months. I just bought the 250 at best buy to use temp. Boy was that a backwards step from a 280 to a 250.....What I am thinking is a gtx 480 and bump up to 6GB of memory. Also, should I ditch Vista and go with Windows 7 ? How much of a headache would changing OS be? Never have done that.......... Any advice would be appreciated.Lamar Wright