So I just spent a little while tweaking FSK to run well on my system. There are a TON of guides out there for tweaking FSX but none of them seem up to date. Basically 90% of the info in them was not applicable to my machine. I thought I'd post what my setup is as it may help someone with a modern machine trying to set this thing up

My rig:
i7 950 overclocked to 4.0GHz
ATI HD5850 GPU
eyefinity running 5760x1080
Win 7 64bit
FSX Deluxe SP2
I attached my config. Here's the items that made big changes to my frame rates. Every setting affected it a little but these had MASSIVE impact on my frame rate. Unless otherwise stated my settings as those recommended in the online guides.
Settings - Display - Graphics
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DX10 - Absolutely terrible. Graphics flicker, disappear at random and otherwise look terrible. 20fps penalty. It's off.
Light Bloom - Can't tell much of a visual difference but it cost 10+ fps. It's off.
Set anti-aliasing to bilinear and do not select the anti-aliasing box. Force AA in the driver. This gained me 7fps over checking it in-sim.
Settings - Display - Aircraft
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This setting was interesting. I set all three exterior settings to "on" with no frame rate penalty at all. AND it looks great with the self-shadowing! Most online guides say to turn this off. I didn't need to.
Settings - Display - Scenery
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The "terrain and water" section was interesting. Basically I can max out all of these settings with zero fps penalty. So that's what I obviously do. Scenery Objects is where it gets interesting.
Ground scenery shadows doesn't affect frames but it looks terrible, flickering, artifacts etc.
Scenery Complexity. This was a BIG one for frame rates. Each click on the slider cost 10fps. Moving this slider made a huge difference. I settled on very dense as dense looked a little sparse for me and extremely dense didn't seem to add much. However, I can immediately boost my fps by 10 just by turning this down to dense.
Autogen density. Surprisingly this didn't seem to be too expensive at all. Maybe 2-3 fps difference per slider position. I set this to very dense also.
Settings - Display - Weather
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This was a weird one. Setting cloud detail to detailed instead of simple actually GAINED 5fps. Very strange but I'll take it!
Settings - Display - Traffic
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THIS IS THE BIG ONE
CARS. Turn
OFF Those stupid little cars absolutely kill the sim. Kill it. As an example I could make my frame rate go from 75fps to 15 fps just by changing the number of cars displayed by 50%. Turn those little blighters off and you'll be glad you did.
Aircraft labels. Most guides talk about this causing problems but I have zero frame rate penalty using them.
Airport vehicle density. This is quite a hit on frame rates. I set to Low, Medium cost 10fps.
Airline Traffic Density. This really impacts frame rates but not as much as cars. I have Ultimate Traffic and can set this to 50% airline, 100% general and the frame rates are great. This gives LOADS of planes and not much impact on frames unless you are flying over a major airport when it can go down to 20fps.
FSX.CFG
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Couple of VERY interesting things here. #1 hyperthreading doesn't help with frame rates at all. I have hyperthreading turned off on my gaming rig. Turning it on did nothing for frame rates
AffinityMask. This was a big one! For a quad-core setting to 15 (1111) means "use all cores" and 14 (1110) means "use three cores" leaving one free. On my system setting this to 14 was a big improvement over 15. Way less stuttering and the sim ran much smoother. Leaving one core available is highly recommended instead of using all four.
TERRAIN_MAX_AUTOGEN_TREES_PER_CELL=800 This tweak limits the number of trees created on "dense" "very dense" etc. It raised by average frame rate by 7fps.
So remember, I'm NO expert on this just wanted to share my findings and what worked for me and what didn't on a modern machine running eyefinity. Hope this help someone new to get started.
My config:
http://dl.dropbox.com/u/14465749/fsx.CFG