What has worked 100% reliably for me since the release of FSX, is to avoid the tweaking. The sliders in game are the best starting point to figure out what your computer can actually handle efficiently. I run very high settings on all, with a quad core AMD and 4 GB of ramm, with a 1GB video card.
I've discussed this issue with others in different forums too. I have an opinion (and it is purely my own, based on no independant testing or otherwise) that the biggest problems with FSX are caused by the constant manipulation that occures when too many addons are added, and too many tweaks are invoked, and they all start to fight for the limited resouces that FSX is able to call on. We all know that FSX is highly dependant on the CPU, and some manipulation of multicores can help to a point, but I tend to believe that this also becomes a choke point when trying to induce too much out of FSX with too many addons. Memory issues abound because of the 32 bit nature of FSX. And as you've noticed, the biggest baddest GPU or SLI combination may not actually result in a drastic performance increase for FSX. I also believe that a lack of standard file formatting between addons causes problems.
Thus what works for me, is to steer clear of all these bells and wistles, and leave my expectations for FSX as they are. The graphics are tolerable. I can see trees. They may not be the best 3D trees I've seen, but I can sure tell they are trees and I should avoid having them directly infront of my graphical display. I can see runways. They may not be the best hi-def runway textures I have ever seen, but I know that I can land and take off there.
So in summary, my personal preference is not about the many (and some very expensive) graphical enhancments available for FSX. I do use the CH products yoke, TQ, and peddals, which is about as high a sense of realism that I am willing to pay for. With these control peripherals, and the countless buttons and switches, it allows me to practice flying as best as possible from the comfort of my basement. It makes FSX a reasonably decent systems and proceedures simulator. EDIT: By not making myself reliant on the GEX and the REX and the UTX and the NGX and the whateverX addons out there, I find that my eXpectations are well within the intended purposes that microsoft released FSX on. An application of the K.I.S.S. principle?
Computer specs wise, I was aware of what could adequately handle FSX and purchased my last computer with this in mind. However, like the OP, my computer has more primary purposes, and FSX is allowed to run so long as it behaves itself, which it has done so for 6 or 7 years now.