That's normal. You or the installer deletes them, you restart FSX, he compiles new ones and the folders are there again. But their contents then differ. That new stuff are your new shaders then.
As said, if those new shaders and the low settings didn't help, there's no holy grail left anymore. Maybe you can borrow a more powerful card from friend to check the influence there. The mentioned GTX460 or a HD6870 might help a lot.

FSX first of all is CPU limited, but those cloud thingies actually are more bound to the graphic card and her bandwith and overall GPU performance. I know that the advertising of your card might explain some game capabilities, but they are very, very limited there.
So if your CPU isn't too weak, maybe a card update helps.

Maybe you can install some lower resolution clouds or get rid of the HD ones (if e. g. REX has installed those at your system). This should help a bit.
I'd search the Avsim library or so or would use REX with a setting of low resolution clouds to test this.

If you reduce the overall texture resolution with the slider in FSX, you will also free up some space, but lose quite some image quality. You can test this though.
One notch below very high at 'Global Texture Resolution' or even one step further down does this. find that slider at the Graphics setting page in the FSX settings.

I don't know that Dell system.
But if it has a free PCI-E slot (which it should have when removing your current card) and can deliver the case cooling and also power needs of the new card, it should work.

Maybe some hardware expert from your friends can help you there, checking this highlighted stuff before buying.
The more powerful cards produce a lot of heat and need much more power than your current card, so a closer look is recommended. If Dell e. g. used a small power supply, you won't get a stable system there.
Otherwise, a shop has to help, but will need money for this.

I think that AMD recommends a 500W PSU for that card and I don't know if Dell included such a thing. The case cooling also needs some 'flow' to allow a safe operation which also covers the noise produced.
They usually recommend too high PSU values there, to be on the safe side. But let those decisions be up to a guy with some hardware knowledge, when looking at your system more closely.

But maybe try those replacement clouds (and slider settings) I spoke of. They are cheap or come at no costs and might help a bit, before buying new hardware. 

Well, if anybody does know the link, notify me, but in the meantime, I've found this: http://simviation.com/1/search?submit=1&keywords=oscar+cloud&x=0&y=0 It is meant to provide less gpu-intensive and more realistic-looking clouds. However, it does come with high-res clouds if you have a good graphics card.

I'm now getting 20FPS and 15FPS in metropolitan areas--all sliders are high or max with the exception of clouds. (Personally, I think those are good frame rates. Some people demand 60 FPS, but I don't notice any major difference after 15-17FPS).