FSX is not about GPU's, it's about your CPU. The real issue here is a lack of CPU horsepower. FSX frame rate scales nearly 1:1 with CPU clock rate, and a Phenom, especially at 3.0 Ghz just isn't capable of giving you the performance you need for FSX.
Unlike most "gaming" titles, FSX is a "simulation" which is very heavily CPU bound, but very lightly GPU bound. As such, FSX frame rates do not respond to SLI or CF configurations. Furthermore, since FSX performs better with nVidia drivers, any mid-range nVidia based graphics card works well.