bro, i just upgraded from a Pentium 4 to a Pentium Dual Core E5300 3 weeks ago...
Dual Core E8200 or Quad Core Q8400? Both are solid choices, but it honestly depends on what games, programs and applications you are going to be running.
I upgraded only for gaming purposes, and on Left 4 Dead I get over 60 FPS and I am completely happy with that, but then I installed GTA 4 and I am stuck between 20 - 25 FPS.
So, if you use stuff that need more than Two Cores, definitely get the Quad.
The Quads are a safer bet by any standard anyway. I advise the Q9300 or the Q9400.