That's a tricky situation with the MONO.SIM error! Before diving into ATI Catalyst or tweaking the config.ini, maybe try enabling texture compression directly in the IL2 setup menu. It's the simplest and might just do the trick. While you are at it, if you're looking for something less graphically demanding while troubleshooting, some fun can be had with
io games; they can be a nice distraction without stressing your GPU.