As Furure_Pilot was saying you do need a powerful computer to run FS2000. A P3 450 minimum. Celerons are nice but they dont have the processing speed to keep up with a program as advanced as FS2000, even the latest celerons. You will need a minimum of 128Mb of RAM as well and a good 3dfx card, Voodoo 3, Geeforce, TNT 2, any of those will be more than sufficient for FS2000.
Ok, I've covered the PC side of things, now the updates that are available. FS2000 is pretty much limitless when it comes to addons available through and other various web sites. People out there are striving to make flight simulation as realistic as possible. The downside to all these addons is the more you download, the more hard drive space you need and the more RAM you need to deal with all the extras.
To give you an idea what your looking at, Im running a P3 667 with 64Mb of RAM, 16Mb Riva TNT graphics card. FS2000 uses at the most just under 1Gb of hdd when installed, my version is currently sitting at 2.61Gb. With this setup FS2000 runs smoothly but it starts to run into problems when processing detailed scenery. The program will stop for 30 seconds and re-buffer all the scenery. This will happen every 10-20 minutes or so and can become quite annoying because more often than not it will occur just as Im about to land somewhere and will result in a rough landing just about every time
Hope this helps