I started FSing with FS98 way back in the days. That was not much more than flying into Chicago buildings (not intentionally at first

. Since a recent job-change I had more free time on my hand, and I happened to stumble upon a discount FSX at about the same time. So I started FSXing about one year ago, having skipped FS2000, FS2002, and FS2004. I joined IVAO, and now I'm hooked! I did not feel like "taking a step back" to FS2004, even though my PC was steaming and puffing under the FSX load. I started collecting addons to make things even better. I must admit that it takes a little more effort to find what you are looking for, but if you do you'll find that there is surprisingly much available. I do use payware addons when I think they are worth the doe, which makes finding stuff a lot easier, of cause. But I prefer good freeware, that needs no explanation. The other day I tried to make a backup of all the stuff I gathered in this last year, and I needed 3 DVDs already!
Anyway, even though my PC is flying at 100% all the time, it's the addons that make me stick with FSX. Especially the payware ones that I bought for FSX only

. Also, I like to fly VC, and the looking around is different in FS9 than it is in FSX. That's just a matter of getting used to, but those few times I did get to fly FS9, that was the one thing I kept doing wrong...
So for me, the addons are the main reason to stick with FSX. I know the same goes for others that stick to FS9. An in the mean time, I am saving up for a more powerful PC

Oh, and to answer Dale's question: I think 20 fps in FSX (which is somehow different from 20 fps in FS9 [huh?

]) is sufficient. Of cause, the higher the better, but in my experience, anything above 10 fps is "flyable", and 20fps + is comfortable. I have my fps "locked" at 20 fps. If you don't have any addons yet, and are willing to put some extra effort in obtaining them, I would advice you to stick to FSX. You'd be "future proof" with that choice.