It all depends on you.
If you are happy with your current system and it does what you want to do, then why get rid of it?
On the other hand if you feel that it longer performs to your expectations then by all means go ahead!
Personally, I would wait until at least Q2 2007 when Intel refresh the Core 2 Duo lineup. You can get the Core 2 Duo E6850, E6750 and E6650 that take advantage of the 1333MHz FSB. Or, look out for the E6320 and E6420 that are basically the E6300 and E6400 with 4MB of L2 cache and will apparently cost the same as the models they replace!