I run 2002 Pro on a P2-266, 128 RAM, with a $40 Wal*Mart Geforce2 generic brand, and frame rates are fine if you turn off dynamic air traffic. I can even run with dense scenery complexity @ 800x600. Frame rates are excellent in IFR (less stuff to render I guess). Virtual cockpit is no good, however (It's lame anyway I think).
My system is extremely lean, however; if you don't stay on top of things you probably have quite a few programs running in the background that you don't know about. My P4-1700, 512 Mb machine at work is slower than my home machine because I do not have the priveleges to remove all the fat. I don't run anything in the backround at home (No firewall (dialup), antivirus (more harm than good in my opionion), quicklaunch stuff, etc...) These need to be cleaned out and turned off to get decent performance out of an old machine.