you would find a huge uprade in proformance in the quad core and 4 gigs, BUT

I am not sure where you are, but my computer is somewhat like urs (was), and I was smooth rolling 4 up to 70 till i hit northrend where it was a little choopy but it was smooth, just not visiaully pleasing, but then dalaran and bg's and bigger questing areas came, and they just died/froze up. I woudl truely want you to consider throwing down like 400-1000 bucks to make ur system run really really fast even in dalaran and BG's when you hit 80

Hears what Im talking about

Intel 920 CPU
intel extreme motherboard
12GB or DDR3 corsair memory
a at 1GB memory video card
some cooling for this awsome rig
750+ watt power supply
=
900-1100 bucks
=
some insane proformance

I am not sure about ur budget, but if you plan on arena and all that I would do this, but if not maybe some small upgrades just to make the whole process managable