Don't be wasting your money purchasing FSX if your machines arn't fast enough to run it.
Microsoft state on the back of the FSX DVD case the minimum specifications to run the game which is absolute crap, them specs would barely load the dam game up nevermind run it. If you want the best out of fsx you need a fekn fast machine, FSX will gobble your memory if you don't have enough!
Basically your looking at:
A good Dual core processor or Quad Core, Not sure how the latest cores work with fsx.
At least 4GB of Ram
ATi Radeon HD 4xxx series or NVIDIA GeForce Graphics card
A good size hard drive, 500GB is good thou not required.
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Anybody looking some decent FSX helicopter addons have a look here:
Nemeth Designs Development Group - Products