G5 is the definitely the best IMO. Inifinite customization options for every plane, so you can make any plane fly how would you like, great graphics, fun mulitplayer, etc. Runs great on many machines, as long as the machine is not ancient. Download the demo to G4, and see how well it runs. Also, stop by and visit the Knife Edge support forums, tons of great info there.

Here is the link to the G4 demo. It is simply to see how well it runs. You can certainly try to fly with the keyboard, but that is not really the purpose of the demo.
http://www.realflight.com/free-g4-demo.html

If it runs great, get G5! You will not be disappointed.