Speaking of quads, I have a T3400 with Q9300. I also have some other computers with E8300 C2D.
For trading, I see virtually no difference... maybe a slight difference in loading-up apps to get started. After that, nearly everything runs from RAM and the CPU sits mostly idle during the trading day.
I recently did a fresh install on the quad and also had to do an update from a several-month old image on my Vostro with E8300. I noticed the Vostro to be quite "pokey" at doing the updates.
The computer's speed has to do with more than just the CPU. A good mobo adds to performance.