i am going to overclock my gtx580 so hopefully it will give me a bit better frames

It seems to cope with ASE and max cloud coverage even at stock speeds. When the frame rate drops it's the CPU that's struggling, so a GPU overclock won't make much difference. Don't get me wrong though, frames are 30+ almost all the time now. It's just occasionally around large payware airports, with heavy aircraft ect, that things drop down to the teens. For example, just landed at UK2000 EGLL xtreme with the PMDG 747, ASE, LondonX and FSX sliders on full. Frames approaching the runway and whilst on the ground were between 15-30. Similarly was flying around FTX pacific northwest in a carenado earlier and getting 20-30 when over the most dense parts of a city.

do you have the light bloom setting on what about traffic settings

No light bloom, zero traffic. These CPU/GPU's are great, just not quite perfect for FSX.Really though I think in answer to your original question in this post, the 580 offers me about a 5-20fps improvement over the 280, depending on the given FSX situation. Easilly adds 20fps up in the clouds, but on the ground where the CPU is working the hardest, it adds very little. Worth the price though I'd say, especially if you're using it for some nice DX11 gaming as well as FSX. Crysis2 is unreal now!

It seems to cope with ASE and max cloud coverage even at stock speeds. When the frame rate drops it's the CPU that's struggling, so a GPU overclock won't make much difference. Don't get me wrong though, frames are 30+ almost all the time now. It's just occasionally around large payware airports, with heavy aircraft ect, that things drop down to the teens. For example, just landed at UK2000 EGLL xtreme with the PMDG 747, ASE, LondonX and FSX sliders on full. Frames approaching the runway and whilst on the ground were between 15-30. Similarly was flying around FTX pacific northwest in a carenado earlier and getting 20-30 when over the most dense parts of a city.

Thank you for the useful input. FSX is so CPU dependent, we might have to wait for years to see the 15s turned to 30s.

Absolutely. Both those situations were really at the extreme end of the FSX spectrum though, so really the sandy bridges very nearly have the sim nailed. Maybe in a years time with the Ivy's and GTX 600 series, FSX will finally be beaten into submission!