Captain Chaos 747,
Have a look at this:
http://www.simhq.com/simhq3/hardware/reviews/p42800/
If you search this site you should be able to find more benchmarks I think.Take a look at their forums as well.
Have you tried Falcon 4.0? With the latest Superpak it`s a lot of fun!
With a 2.8 ghz they get 27 fps. all maxed out.
FS 2002 is probably the most CPU consuming flight sim out there.
But you should do pretty good with your system.
ORAC`s link is valueable for tweaking.
OH Captain,
You did not mention type of graphic card.
Retailers often try to screw you up with a cheap one. Even with a good CPU the card might be a bottle neck. I`ve heard that the 4600 Geforce 4,128 MB is good. The Gf 4 MX has a bad reputation.
The extra pounds for upgrade will be better in the long run.