Well, the graphics card GPU has very little if anything at all to do with working the AI functions. AI functions are not graphics. But with alot of AI aircraft in the scene, it could bog down the graphics performance because the vid card must render more on the screen, along with everything else (ie background, details, clouds, scenery etc).

You will have to experiment with the different settings to find the optimal performance. I run a P3 600 machine using a GeForce FX 5200 card, I get very good performance even in congested areas and scenery with lots of AI aircraft flying about.