Personally, I wouldn't spend anything on an upgrade simply to run FS2000 at this time. Firstly, whilst the program is OK, it will only get better with time as the bugs are sorted out and secondly the price of processors that run the thing smoothly will only get cheaper as the faster come on line. Another point mentioned elsewhere is the new gen graphic cards that take some of the load off the processor.
Summary=the program is not that good that you can't wait six months to affordably run it...IMHO