After doing far too many upgrades I think you would be better off moving to Windows 7 than a CPU change. Other areas that pay off are RAM (get 2 to 4GB) and video cards since onboard video while nice enough for some things can be a bit sluggish.
There are other areas that will pay back more than a CPU change.
I have a Dell Dimension E521, dual core, with AMD Athlon 64x2 dual core processor 3600+ 1.90GHz, running vista. My question is can I up-grade the processor itself or will I have to change the motherboard as well? And well the up-grade be noticeable or will I be wasting my time?