In a 2009:
8 core 2.26
8GB RAM
OCZ 250GB
1920x1080 x 2

Using AIC AND general After Effects content I have seen:
4 layers on out 1 and 4 layers on out 2 at 25FPS (8 total layers) no FX
3 layers on out 1 and 3 layers on out 2 at 30FPS (6 total layers) no FX

I have tried the same thing however, with the same size content but content that is much more vibrantly colored and complex (like HD footage of nature) that caused frame drop.