but do i have to expect less FPS in FSX if i use 3 monitors at the same time with ATI Eyefinity than if i use only one monitor?
Yes, you will take a fps hit.
I am running 5040x1050 with a 1GB GTX285 and TH2Go.
E8400 at 4.0GHz and 8GB of ram.
It does okay and I have never had the out of memory error.
In Game settings and GFX driver settings:
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I haven't done any of the supposed tweaks I have read about. Never really had any boost from them in the past, so I didn't do anything except a clean install of the game a while back. Runs super nice, looks great and never gives any trouble.
I mainly fly GA planes under 5000 feet for the most part and from smaller airports, so I have the heavies off. Lots of road and water traffic for scenery.
EDIT:
I forgot something. One tweak I did do, but really doesn't hurt visuals much and will increase frame rates.
TERRAIN_MAX_AUTOGEN_TREES_PER_CELL=2800
TERRAIN_MAX_AUTOGEN_BUILDINGS_PER_CELL=3500
Best FPS boost going of anything I have tried, EVER. It is the one and only tweak you really need to do. All those others you read about really don't do much of anything, unless maybe your system was a mess to begin with.
And there are still plenty of trees and buildings to be seen

I am on Vista Ultimate x64 SP2. My video driver is 182.50 (the best one I have tried, for my card/system).