I am a total NooB to MS Flight Sims, and I have purchased the 2002 addition beacuase it came with a reduced cost Sidewinder Precision 2.

So now that I have this, is there an upgrade so I can get the Pro Edition? if not, why not, surely I don't have to go out and buy the full game again...

So is there much difference in the pro, is it worth it?, or should I just wait and get the 2004 Pro. Or can I d/l the extra planes that came with pro. I want a good twin engine to train in.

What a load of rubbish !      If you have some idea as to the Twin-engined job you fancy, search this site, or even AVSIM and download what you want.    The Pro version is certainly a better buy and this subject has been beaten to death for the last two years on this and other forums. Having said that, unless you are a FS nutcase, you can have immense fun with what you have, as do probably half the people on these pages.     Certain planes will be barred to you but many of the designers have done one for the Pro and one for the Standard.     If you have any specific queries about downloads or other areas of expertise there are lots of friendly folk here who are experts in their field and will give you any assistance.
Good luck with it, stick with standard meantime and wait for the next version.