That's a complicated question and almost impossible to answer, especially without knowing exactly whats in your computer. You are in the same boat with many of us, we want better performance but we don't want to by a new super computer to get it.
1. If you try to change your processor, you will probably have to get a new motherboard and all new RAM. $$$
[Major upgrade, may be cheaper to get a new computer est. $1000]
2. You could by a new, dedicated video card.$$
[Probably you best bet, price depends on choice of card, est. $300
3. You could buy more RAM(Don't know how much you have).$
[Always good to have more RAM, est. $100 per GB]
For FSX, you want the fastest processor you can afford, good video card(preferably one that can upgrade to DX10 when it's available) and 3 GB of RAM. So if you get a video card and 1 GB more RAM, that should be good and will run you about $400-500.
I am no computer builder, just my opinion. Some of the people here actually build their own computers and are a better source of information than I. 🙂