It is definitly NOT worth the extra money for so small increase of performance!!
You also have to consider upgrading your graphics adapter for PCI-X one.
I, for one would never do this!
In june, AMD is releasing new socket AM2 and only then it would be wise to upgrade. Socket 939 is soon obsolete. But then you will have to change your RAM for DDR-2.
So, my advice is... wait till late summer and then switch to 939 and some dual core processor (the prices will drop when AM2 socket is released). Performance wise socket 939 is good for a year or two at least.
Of course its your money and you're the one to make a decision