There's not going to be a big difference between the dual core to your current one. The quad core could make things a little easier for the computer, but i doubt you'll notice that at the low clock speed either. I'd just get yourself the better drive and extra RAM. That's where you'll notice the biggest improvement.
3.8GHz i5 760/8GB RAM/Samsung 1TB Spinpoint F3s.
Shine WEB45, Vintage Metal Axxe Wraith, Stagg, Aria Pro II JS, Yamaha PSR-275, EKO Acoustic.
Studio Projects B1.
Studio One, Reaper.
Metal, Jazz, Ambient, Trance.