Flight simulators need more CPU and RAM than GPU.
Not quite.
I've been running a lot of combinations, from very weak CPU with reasonable GPU (P4 2.6 GHz with ATI 4670 AGP) to good CPU with very weak GPU (i7-920 with Nvidia G210) and regardless which part is too slow, it will always ruin the full rack. "Each to his own" yes, it always depends on what you're planning to do with that particular machine.
I'm running a well balanced system too (i5-2500K with Nvidia GTX-550Ti) and that one runs IL-2 about 20 times as fast as the other two mentioned above.
As for the ATI cards, I've been running both ATI and Nvidia cards on the very same system several times.
ATI always gave me a headache with their drivers, the missing option to run different windows with different 3D settings in parallel (e.g. Anti Aliasing, on ATI without special "tricks" you have to set this system-wide, whereas Nvidia applies this setting per window, useful when switching between game and Java IDE window during mod development), untraceable crashes down to whole system non-bootable due to boot-time BSODs and the like.
I never encountered any of those issues with Nvidia. The worst I see with Nvidia cards about once a month is that some system for some reason (don't ask me why) at a given time puts a notification into system tray that the GPU driver crashed and had to be restartet - lol I didn't even notice that: The Nvidia driver restarts without any glitch where ATI drivers used to crash the whole system without further notice.
The backflip is that generally Nvidia cards were a little slower than their ATI counterparts on same price level and
if the ATI card ran fine, the image quality was real stunning, quite beyond what I managed to squeeze out of Nvidia driver settings, this particularly comes true to shadows and Anti Aliasing.
So
if your ATI card runs fine, don't touch anything and consider yourself a lucky fellow.
If not, better opt for an Nvidia card if you don't like fiddling your system, especially IL-2 likes Nvidia cards much more than ATI cards.
Best regards - Mike