RE: G4 Real Flight is great!
Just an F.Y.I. I just got RF4 (from RF2) and I was quite worried that my old computer wouldn't be able to run it at all. I was pleasantly surprised! It automatically configured my settings to optimize speed, I believe most things are pretty much set to medium and I pretty much stick to Photofields.
My system: AMD 2.5ghz, 2gb ram and if I remember correctly, a 128mb vid card (6800gt??).
I'm going to upgrade the system in the near future, but it's not as much of a pressing concern as I thought it would have been!
Pete