*Points at Celeron*
Sorry, but Celeron's aren't really built for games (built for more like Word Processing or ect).
Celerons, well, lets just say you shouldnt expect to run ANY game on it well, espeically if it is 3D based from my experience.
But 5-6 FPS does seem abit low, on vannilla FS2 from the year 2000, I expect you should ATLEAST get 10 (which still doesnt cut it).
Well, the solution is easy, replace your Celeron.
Unfortunatly, from what I have seen, Celerons are really only featured on Dells/HP's/ect (aka any company that makes computers. Always the "cheap" computers that PC companies sell seem to use Celerons), and if that is true, you will probilby have to buy a whole new computer (or all the parts that need to be replaced to support your new processor, AKA ususally means a whole new computer).
Big name PC companies tend to make it so its hard to upgrade thier computers, which is why I doubt you can just buy a new processor (assuming your computer is a Dell or somthing else like that).
I could find a link to a $172 AMD 2700+ full PC that i found on a website if you want, which will run FS2 at amazing speeds (but wont cut the cake on modern, high detailed games, unless you upgrade it. Even if you upgrade it to the max that it's motherboard supports, you still wont find yourself playing modern-day games at max possible speeds. Buts it great for a general-games computer). The only problems is that it only has 128MB of RAM (i belive you can just use your computer's RAM and put it into the new one), no video card (same for above, just see if your VC and put it in the new computer), and it runs Linspire (aka a crappy O/S that you can download for free). So, if you buy/get new RAM, a video card (such as the one you have now), and Windows plus a HD to put it on, then it will win FS2 (and many other games) excelently enough.
So, your problem is your Processor IMO. Celerons suck, for pretty much anything except general web browsing and word processing.