From my experience fps should always be set to 'unlimited', because the limiter is a really crappy. I was having 30 fps in some scenery... didn't realize why, when other, similar sceneries yield much better fps so I changed to unlimited fps and ta-da 20 fps increase, problem solved. I also read that setting it to unlocked does an undocumented change, which is displaying the scenery with greater distance and curing possible blurries.

I tried unlimited seems to work the same as locked in terms of qaulity. I try all types of settings all the time water run it at 2 mid as I can't stand flashing water. The cost of fps like 3 to 5 no biggie when you get 40 to 60 outside cities and turn autogen off. All try external lock with nividia and see what happens.

On my system it depends on the aircraft I'm using funnily enough. For heavier resource aircraft like the NGX, AAX, QW146, etc, I find that keeping the FSX slider to Unlimited provides the most frames and best smoothness. On contrast to that, I find when I use less intensive aircraft like the MJC Q400, anything from A2A, Carenado, etc., that limiting my frames at 30 gives me a constant 30 and the best smoothness. With the lighter addons frames to Unlimited gives me choppiness.

 

It seems it's very system dependent, and also depends on what tweaks you have, if any, and if you use an external limiter or the FSX one. I don't play around with tweaks. From providing support, they seem to cause a lot of issues when users just try and dive into them without fully understanding what they do and how their own system works. HIGHMEMFIX and AffinityMask are all I need in my experience. Best piece of advice is pick two or three test places with different levels of complexity, and different aircraft with different levels of complexity, try the internal limiter at different settings, and maybe try an external one like Nvidia Inspector, and test out what works best for your system.

Prior to my current hardware, I'd run many, many test flights trying to figure out the optimal settings (since I run an ATI card, for some tweaks I've been on my own) for FSX.  I used to switch between internally limiting frames to 30 when using demanding addons and running unlimited with smaller GA. Occasionally, when trying to fly where the system was having trouble maintaining 30 fps, I would run unlimited frames for that bit of extra performance.

 

Now that my system is more capable, I find that I have the best overall success with frames being internally locked at 40 all the time.  95% of the time, the sim holds that no problem, and when it dies dip at certain airports, it rarely drops below 30.  I don't like running unlocked anymore, since when there's not much load on the sim, it tends to try to do too much too quickly and I get occasional stutters that coincides with the internal fps display starting to spike.

 

For photoreal like Blue Sky Scenery, it needs to be limited for the smoothest performance for me.  I can internally lock at 60 just fine, but since I can't really tell the difference between 40 and 60, I just leave it at 40.

Great great thoughts guys.. Running unlimited causes spikes when flying GA, I experienced it myself flying Alabeo's Su-26. Locked 45 fps solves the issue. Well, definitely not always 30 like today's standard as 30 limits todays hardware capability. I don't fly heavies in FSX that much so I can't tell but I guess flying heavies in complex airports don't cause spikes because there's nothing left to be limited,the fps is scary

I've tried locked at 20, 25, and 30 using the internal limiter in FSX.  I've also tried setting FSX to unlimited and using the external limiting program as well as trying the EVGA video card limiting program.  I have found that although my fps will jump and dive, overall smoothness seems to be better with everything at unlimited.

From FSX it's set unlimited, I'm using external FPS limiter to keep it at 30. Seems to work the best for me.