If your planing to buy a PC, the best way of doing it is putting the thing together yourself or having a store put one together for you. This is because as time goes on the compononets will become outdated(slow) quite quickly and wont be sufficent to run the newest games at an accepteble speed, so you will need to upgrade them....something that is often not very easy with computers off the shelf. Do some reaserch on the net and find out about the different components that will suite you. I recomend a motherboard with an AGP slot + 4-5 pci slots(this is where video and sound cards go, eg Voodoo3), a P3 or Athelon processor and 96-128 megs of ram.
Try posting in www.3dfxgamers.com/boards.asp, the guys there will really help you out.
As for flight simulators, your best bet is FS2000 or FS98 if you choose to buy a slower computer(400mhz or less). You will need at least a 500 mhz cpu to run FS2000 and a good video card, which you will need for either game to make the graphics more realistic( the diffrence is like night and day); I recomend the Voodoo 3 2000 PCI or AGP.