Caporegime
Where for £15?
Make sure you have a mother of a machine to run it. I'm getting 20-30FPS av on my rig with medium/high settings. Otherwise FS9 may be a better bet.
Mother of a rig?
I'm running a heavily modded (i.e. add on scenery planes etc) FSX install with an A64 3000+, 256MB 7600GS AGP & 2.5GB RAM.
FSX and FS9 both run extremely well on low ish (compared to yours!) spec PC's.
I get smooth play with my FPS locked @ 20.
Tweak, experiment, tweak some more, you'll get there.
http://ops.precisionmanuals.com/wiki/FSX_FPS_Guide
http://64.233.183.104/search?q=cach...Guide.pdf+fsx+tweaks&hl=en&ct=clnk&cd=2&gl=uk
http://www.flightsimworld.com/forums/index.php?showtopic=92583
Just a few guides to FSX tweaking - Many many more out there.