Hawkeye,
I have a 1.7Ghz P4 with a 256MB graphics card and I get very poor performance out of CFS3. You should be able to pick up a very cheap CFS1 and CFS2 these days, CFS1 is fun and CFS2 possibly the best combat flight sim out there. There are people here (you know who you are Katahu

) who will reccomend IL2 and that will run reasonably well on your machine.
As for the FS2k4 issue, I have better frame rates in this version than I had in 2k2 and frame rates aren't everything. The GPS is way better and so is the ATC, I was offered a "circle to land approach" the other day, it blew my mind LOL. In addition the graphics of 2k4 look better at 1/2 to 2/3 settings than 2k2 looks at full settings.
In all things it is a balance. You will not get video smooth graphics and screen shot competition quality at the same time, there will be a trade at some point.
Am I making sense? It's 3AM..... :-/
Will