A good computer to run the simulators?
you could just spend a bit of money on a new graphics card and drop it in. Should be good enough then.
Don't even bother with ram, you get no noticeable speed increase, but yes if it has an extra ram slot then you can fill it.
Not much on the horizon PC wise, the Geforce FX is due out as well as a faster P4, but effectively it will remain pretty much the same.