This is gonna be fun. Here it goes:
For many and most current software and games, two 3.06GHz cores will perform better than four 2.53GHz cores. We (I) keep in mind that almost NO software is yet written to take full advantage of dual-CPUs, much less quad-CPUs.
On the other side of the coin, if you are running many apps at the same time, a quad-core would be able to balance all those apps among more cores than the dual-core, thus making it more responsive.
On another side (3rd?) of the coin: your comp will go further with a quad-core installed - reselling
I recommend that you review each requirement of your apps and games, read and research if they are better with duo or quad. Balance out your usage, then go for a decision.