Fozzer,
I read your post with interest, I too have a similar machine which has served me well over the years and still does, like you say for average browsing and day to day use, I have also been curious to the answer and sadly I'm still asking the question.

Now, although I don't run FS on this machine you could argue that I'm lucky to have FS9 installed on an additional custom made box albeit built around 2004-5 and based around an athlon xp2600+ and a geforce 6600, comparable to the P4 somewhat but with the gpu giving it that extra edge FS9 runs pretty well to a point, however I feel I've flown it to death even though as good as it still is and with the life there is still left in it.

So the plot thickens, along comes Santa and finally leaves me a copy of FSX! Custom build box here we come, sadly though "computer says no!" so to the trusty bog standard optiplex P4 and she's as happy as larry with activation and all but I know what to expect.

Well, I'm familiar by now with FSX's demands even on a high spec PC and it didn't disappoint me, all sliders all the way down it isn't very pretty at all but I kinda new that.

It's highly unlikely I'll be ever able to justify the cost of a PC to get this to run properly but maybe here's a question you could answer.
If you were to acuire a second user or custom made machine and seeing as though FSX apparently likes to munch its way through CPU power any ideas as to what would be a half decent older processor that's 2 or 3 or maybe I dunno, 4 years old that would meet FSX half way and at least make it flyable? in other words FSX was released a good few years ago now, what would have THE processor at the time must surely be more affordable now.

All the best.
Happy new year everyone.