I agree with Rock on going to 64 bit. You run out of RAM ability and while 32bit will recognize 4gb of RAM it really only adequately uses 3gb ram so you top out.
I also agree with Rock that software will only continue to grow in working with and utilizing multiple core hardware. Thinking about applications that are current or legacy is only present tense thinking. You want to keep this computer for 4 years so imagine how many iterations of applications you will get that start maximizing those dual and quad cores.
Heck our software updates for programs like AutoCAD and other engineering work routinely come with versions to run both 64 and 32 bit now. Where as just a year ago they didn't it was an either or. So companies are recognizing the shift in technology and that will only continue.