Sorry Dom, but I feel the urge to go off topic for a second to respond to Murray's response:
[off-topic]
To put things into perspective, my specs:
P4, 3.2 MHz
2048 MB DDRII
ATI X700 256 MB
ATI P5Q Premium motherboard
~350GB HDD space in total, of which only some 5 GB free (I need to clean up again)
Twas a super duper system at the timme I bought it

Only that was a couple of years ago ...
[/off-topic]
FSX indeed gives remarkable results, even with a bare installation (no add-ons). Especially clouds and water are a huge improvement to FS9. So, top-notch specs are not required per sé. Also, FSX looks smoother at low frame rates than FS9, so it doesn't require 40 fps to be flyable. I accept 10fps as well enough, and I have set my max. fps to 20.
With the PMDG Twotter, and the freeware Himalayas addon scenery, I fly at about 20 fps in my Indian High Mountains Pilot Skills Tour (See IVAO forum / pictures). So even my low specs allow for some (well programmed

) add-on aircraft and scenery.
All that beiing said; it does help to have more heavy spex. Especially if you want to have high autogen density, or lots of AI (or online-) traffic. And the situation remains: given one spec, FS9 will outperform FSX in fps, but needs more add-ons to get to the same visual quality (which in turn "cost" fps).
All in all quite a lengthy debate to end up where I started; Imho it is a tie between FS9 and FSX at this moment.
Happy reading; I now leave the stage to others...

Eric-Jan