SP1 improved things a lot in terms of performance, but FSX is still a lame duck on contemporary hardware.
I have FSX on a dual core system with 2GB RAM and a 256MB graphics card. If I spend time tweaking and messing about with the settings in FSX I can get pretty good performance with the default aircraft and scenery, but when I tried throwing a complex aircraft in (the FeelThere Legacy) it sucked my framerate down to well below 10fps. It seems if you run FSX on current hardware there's not too much headroom for advanced aircraft or scenery addons. This might end up biting the likes of PMDG and Level D on the backside as they have abandoned FS9 in favour of FSX for their future products. I really can't see many people wanting to pay even higher prices (check out PMDG's 35% price increase) for an aircraft that runs at 10fps on current hardware.
I've read reports from people running absolute top of the line systems, and even they say their FSX performance is't too great. Maybe in a year or two we will have the hardware to run FSX and have the horsepower left over to throw in a sophisticated airliner addon or detailed airport sceneries, but that's not the case at the moment.
If you like to fly relatively simple GA
aircraft, then assuming you have a dual core system, 2GB RAM and a decent graphics card, FSX will probably do the trick. If you have a slower system, or you like to fly the more advanced airliner addons you are better of staying with FS9.
Then of course there some of us who think that FSX looks a little "cartoonish" at the moment, and FS9 loaded up with addons like Ground Environment and Flight Environment actually looks better and more convincing.