In that case, I would upgrade to the 6800 GT man. If you are a gamer, you'll probably see a much higher increase than upgrading the processor (not even upgrading but replacing).
Even with a FX-55, if you are using a 6600 GT, I would presume that it would be difficult to reach anywhere close to 4000 on 3Dmark05. Where as, with a 6800 GT you could easily
top this (on a 3000+).
EDIT: If you are not a gamer, you should tell us since there are many other components that might affect the particular application more - if it is not gaming (CPU, HDD, etc.). But, much like most people here, I am gonna assume that u are a gamer.
.. And since u mentioned a couple of vid cards in ur initial post, I am almost sure u are a gamer