I got a killer deal on a Dell with a quad core in the Dell Outlet store (Optiplex 755 small form factor). After using it I think you are better off with a faster dual core - not much software has any clue what to do with more than two cores.
I have the Q6600 running at 2.4GHz; but right now it's running at 1.58GHz, sometimes even slower - it's just not that fast. Even when running Photoshop or Elements. I'm using XP so the OS is not really optimized for the 4GB of ram or the four cores either but I have not tried Linux yet.
So at this point it seems to me having a quad core is being a wee bit ahead of the curve and the world is not really ready for it yet.