FSX I don't know, perhaps fs2k4 was more capable with mods while running nice and smooth rather than grinding your system down to a halt. like FSX did.
FSX was a hardware beatch when it came out, no doubt about that, but nowadays, on my E8400 @ 4GHz rig (built from components on OcUK I might add) I can get simply amazing framerates in Vista 64 SP1. Compared to some of the amazing hardware rigs I see users here with, most would have no problem running FSX at very high settings, so long as they are willing to do a little tuning.
I'd highly recomend NickM's tuning guide
http://www.simforums.com/forums/forum_posts.asp?TID=29041
He's bang on the money there about pretty much everything to get FSX Flying, dont agree with him on the Nvidea stuff, the newer ATI 48** series cards do not suffer the slow down in clouds he mentions, but I've followed his guide and installation procedure taking in to account my ATI 4870's in Crossfire (Crossfire disabled for FSX) and I've never had a more leaner meaner FSX system in Vista 64 SP1 than I have just now, and I've tried it on Vista 32, XP32 and XP64.
Honestly ... I know many friends who moaned about FS9 when it was released because their FS2002 Add-ons were so prolific and could not go fwd to FS9 (better compatibility than FS2K to 2002, though) we could argue all day about the rights and wrongs and what could have been, but this really is the hear and now, this is what we have, and its very very good.
I've been doing FS since FS2, and followed all the other options as well, like the Flight Unlimited series and so on, we are always going to see users with rose tinted glasses, or those that simply wont or cant upgrade their hardware.
My system is honestly quite modest these days, but it fair plays FSX really well.
I say for those that have been sitting on the FSX fence, now is a good time to jump over.