FSX ran ok on my laptop (2GHz, Core 2 Duo with and Nvidia Quadro graphics - bog standard Dell Latitude D630), but the sliders were way way left and the traffic settings were low too.
Got an ok performance but graphic detail was pants.
Over this last weekend I've built a new desktop with a 3GHz E8400 processor, 4Gb RAM and Asus 9600GT 512Mb graphics and FSX runs sweet as
you like. The most impressive difference is in the ground detail and weather. The Africa relief mission is a good benchmark - Lapdog had no ground detail, no weather effects and good visi. New system had trees, appalling visibility and a real cloud ceiling. What was an OK mission is now quite a bit more challenging.
It is addictive (especially when you've got too much time on your hands) and some of the add ons (I use Just Flight - JF Google it) are fun. The Traffic add on is good and Air Hauler, despite having a few logical holes is good for giving you a purpose to using FSX, as I got bored with just flying around after a while.
Still prefer the real thing mind..