I have an AMD 1.5 w/ 512 MB PC 2100 DDR with a GF4 Ti4800 128MB DDR video. I run everything at High and stay between 45-120 FPS depending on the airport and what's in it. I've also noticed that typically, the included planes and airports run a little better/faster than the ones I download on the G2 swap page.

As for flying into the white wall.... Imagine the airport more like a 6 sided box surrounded by a white cloud. The airport and the objects you see would be in the box. It is 3d as you fly around inside it but it has edges (or walls) surrounding it where there has been no object detail entered. When you hit the white wall you have reached the *edge* of the box or flown out of it. That's a little simplified but I believe it explains the general idea. There area also some glitches in some of the airports. Rainbow canyon has a ditch that if you drop into it, you can actually fly Under the board.

Hope that explaines it a little.
Rick