this is only my opinion but I wouldnt like you to rush out without considering what on and why you may wish to spend money so please take some time and then come to your decision.
I think that before you do so please read luckyboys guide and my guide to Il2 optimisation both at http://www.airwarfare.com/index.htm in guides.
these should give you some background.
Wether you decide to upgrade will depend on what you want your pc to do. And what your motherboard can take. And wether you will see as much of an improvement as you want for the money you spend.
If you decide to put in more memory then an extra 512 mb ram should allow the game to run smoother
Please make sure it is compatable with your motherboard and existing memeory. There will be a specification for memory on the manufacturers web site. I always recommend using crucial memory to ensure compatability.
Please take into account wether it is worth while upgrading within your current motherboard or if the money would be better going to upgrading to newer technology.
Hope this helps