Well...
I get very nice FPS in FSX - if I had to say so myself... I have for a long time, dragged porting to FSX - mainly because I couldn't assemble a system, which could run FSX with the level of detail and FPS which I wanted... (I'm quite demanding regarding this!)
I finally ported... I and WON'T go back to FS9!!! FSX offers much more than FS9 IMO... Also, FSDT's sceneries really looks nice! Mainly because the higher resolution of textures and the shaders offered/and used to create waters/wave effects etc...
For instance... I get around 30-40 FPS (smooth framerates) over L.A (or similar cities/dense areas) with Ground Environment Xtreme, Ultimate Terrain X and (of course) Real Environment Xtreme installed! - Using ActiveSky Evolution for weather implementation!
Both night and day... (day in general a little higher). I have ALL settings to the MAX! All the way to the right... Running in DX9 mode... (mainly because some sceneries/addons doesn't support/work well with DX10)... Light Bloom, Anti-aliasing, Anisotropic Filtering enabled. This is typically in e.g Level-D 767, QualityWings 757 or PMDG's 747...
At larger addon airports (e.g FSDreamTeam KJFK, or FlightBeam's KSFO) in dense city areas (with high autogen) the FPS is around 20-30...
Uhh, I purchased all the parts as individual pieces - and assembled everything myself... I payed around 1.200 Euros for it in total... including the screen...
I'm very happy with it! The only thing, I should upgrade soon, is a little more RAM... I'm planning to get 4GB more...
Intel's Sandy-Bridge architecture is just phenomenal!!! I'm really amazed at the performance...