Yes, but -and correct me if wrong- The parts of the OS that a given program must go 'through' aren't on the HDD;they're in RAM aren't they? That's what the boot-up sequence is, yes? It's loading the OS into RAM.
Yes, but not all of it (unless you force it to), and then there's the swap file (unless you disable it (highly
not recommended) or create a RAM disk for it - which, if I had 8 gigs, I would do using 2 or 3 of those gigs). Plus it still has to be loaded into RAM from your HDD. Loading it from a SSD would be much faster.
The primary rationale for my decision to put FSX, instead of my OS, on a SSD was this: unless you optimize the scenery files with a 'name defrag' on your HDD, you're going to get stutters every time FSX has to seek, read, and load a new set of scenery files as you're going along in the sim. When I put FSX on my SSD, those stutters were eliminated.
SSDs are ideal for 'read' operations as opposed to 'write' operations, because a lot of 'write' operations will reduce their lifetime (which wouldn't be a big concern if they were priced like HDDs). The OS does a lot of 'write' operations (especially thru the swap file, although the swap file can easily be moved to your HDD).
The SSD I plan on eventually getting for my OS only, will be about 60 gigs. 60 gigs is under a hundred dollars over here. I can live with that price in return for the extra speed.
You guys got me into buying two SSDs

Just a small question: Is it possible to copy the FSX files to the new SSD and to somehow fix the maybe broken reg files?
My plan is to copy the whole FSX folder to the new SSD and also change the device letter to the same as it was on the FSX/HDD.
Is this possible or do I have to install FSX + addon again?
Thanks in advance
That's what I did. I changed the drive letter on my FSX HDD partition, to free it up, and then assigned it to my SSD after the transfer. Not a single hiccup, and I never touched the registry. (I'm assuming you gave FSX its own HDD or partition, and didn't install it with all your other programs.)