A better core 2 duo would help as well.
I am running a 67 or 6850 3.0G stock and all is running great.
A quad core would be a good thing but FSX really wants GHZ, so if you do get one AIM High.
Also a new Video card can help some things.
This can be installed now and used when you do get the new computer as well.
Not knowing your experience level here is a good guide to setup.
I can tell you that shadows have the biggest effect on performance.
Especially "Aircraft casts shadows on itself"http://www.sim-outhouse.com/sohforums/forumdisplay.php?