I'm running it on a P3 500, 128 mb ram, 32 mb video card (similar to DTA's suggestion), and getting a frame rate of 20 per second in cruise, although if you fly into cloud it'll drop to about 4 per second. Frame rates on landing are usually 15-16.
If you want to check this, hit shift+z twice and it'll display the frame rate in the top left hand corner of the screen.
http://www.simflight.com has lots of useful info on this topic, well worth a look. However it has already been suggested that FS2K is no more than a beta for FS2002 or whatever they're going to call it!
It is a lot better than FS98 when it's running smoothly though, but I wouldn't recommend spending a fortune on an upgrade just for this....spend your money on real flying hours instead
