popatim :
Well you have a Phenom II x4 already, upgrade the ram and GPU and you should be good to go.
Please note that to use more than about 3.5GB of ram you will need a 64 bit version of windows and I dont know which versions of MSFS will work with 64bit Windows.
FSX works great with 64 bit versions of Windows.I am sure that Microsoft Flight will too.
For a new build (right now that is) go with either the Intel Core i5-2500K CPU or the Core i7-2600K (best) for the CPU as they have the highest IPC and best overall performance vs cost.
The performance of the CPU is the most important part of making a great FSX system.
Some say it's 75% the CPU and 25% the GPU.Others state 80% CPU and 20% GPU.
Also get an aftermarket CPU heatsink/fan too as you ideally might prefer to o.c. those CPU's for better performance for FSX.
You can get a lesser expensive LGA 1155 motherboard for those CPU's as there is no real need to get a exorbitantly expensive one.I wouldn't go too cheap however as it's ideal to have a very reliable motherboard.
A Nvidia GTX 460 card works great but a 256 bit Radeon card like the Radeon HD 6850 works great too for FSX.
The 128 bit or 64 bit Radeon cards will have poor performance.
FSX doesn't like cards with multiple graphics chips in them nor SLI or Crossfire as you will actually get lesser performance.
One can actually make a great mid range FSX system on the cheap.You can spend more for a better graphics card like a NVidia GTX 580 but it's not cost effective (will give a slightly better performance though).