I think as our own FSX user community grows (I was encouraged by the number of people we had online last night, biggest crew ever and it didn't include a number of regulars or members of this forum, but we will change that B) ) it would be valuable for us to start posting details of the tweaks or texture mods we have applied, if any, because I know some havent.
This would then give new users a feel for what others have found useful and is working/not working for them, and perhaps make it easier to modify the config file etc.
I'll trawl through my config file and highlight the tweaks I have applied later today.

Edit: My Tweaks.
Edit - Some of these are now redundant post SP1
Let me start by saying this thread isn't here to debate why we should have to tweak FSX to get decent performance from it, its about sharing our knowledge and experience to enhance everyones FSX experience, just like we have done with every version of the sim.
I have done a few things, some aimed at performance and some at visuals. The first thing was Autogen. I did a few things here:
1: Removed the default autogen, this is basically chicken restaurants, petrol stations and fast food shops. It doesn't affect houses, buildings or anything else, just the specialty high polygon shops. It was simple, in the FSX Autogen folder I simply renamed default.xml to default.bak. Did this help? No idea, I had this tweaked before I even ran FS because the reported FPS gain was so good.
2: I changed the density of autogen drawn - this was in the config file.
TERRAIN_MAX_AUTOGEN_TREES_PER_CELL=900
TERRAIN_MAX_AUTOGEN_BUILDINGS_PER_CELL=1000
The maximum value you can have on either of these is 6000, so as you can see I have pulled it right back. I'm still amazed at the level of autogen displayed, as some of my screenshots have shown, so am happy with the result here. Another setting for this is suggested at:
TERRAIN_MAX_AUTOGEN_TREES_PER_CELL=4500
TERRAIN_MAX_AUTOGEN_BUILDINGS_PER_CELL=3000
See how it goers, but this WILL make a big difference if your system is struggling with autogen.
3. Default autogen texture size reduction - I went for the updated textures that halved the size of the autogen textures. This means they load faster and use less ram, but also means they are not as crisp as they could be. I backed up the originals first and will probably go back to them, it will be interesting to see the change if any.
The last thing, and this is quite recent, was changing the Mip bias..
[Display]
MipBias=6
What this has done is sharpened things up considerably. I'm still not 100% convinced I want to keep it, but it has certainly dealt with some blurries I was getting, so worth a look.
You will find this site quite valuable for tips, tweaks and ideas on display settings.
TWEAKS AND TIPS