For software, Microsoft does the job very well. X-plane is OK but the cockpit pictures are not scaleable.
Depending on what you are doing, you don't necessarily need the latest or greatest version. You don't need a fast processor either, but it does like lots of memory, and if you get FS 2004 make sure you get all 4 discs - the fourth has to be in the CD drove and people often foget it's in there when they sell the computer.
Phil