An 30 FPS average is quite playable in a flight simulation. In fact a low FPS in the high teens is still playable. Remember, this is not a first person shooter, where a super high end graphics card will help. The TW series scales quite nicely with CPU speed, and does quite well on my Intel E8400 at 3.6 Ghz.
ATI cards have had some issues with this series, right from its inception. Those issues ranged from texture corruption (RWR screens did not display in the first two patches with ATI cards), to inexplicably slow FPS, when certain graphic options were enabled. I have a "mere" GeForce 9600, and I regularly see FPS well over 70 (when I disable refresh rate synchronization), with all graphics options set to their highest, except for horizon draw distance (which is CPU limited).