From what i've read,multi cores shine when multitasking programs....for example,you can run a demanding game,powerfull video editing software,and ANOTHER game all at the same time and retain functionality.the fourth core would dedicate itself to your operating system.
I've logged respectable time on dual cores,and they don't seem to out perform my single..but they ARE dedicating a core to the op system,so a dual would be nice,AND cheap.I think quads,and sixes are utter overkill for most users.
I'd have to talk to someone who owns a quad core or better,AND is an advanced user who's opinion i trust.
I'm quite happy with my single core 2.4,and prioritizing ONE program.If i run a game,i kill all but the essential processes.My friend and i play rome total war alot,and our video cards are equal,but he swears my single 2.4 outshines his dual core 3.0 on it.He runs vista however,and i XP.He might have some malware slowing his up...hard to say.
I suggest older hardware cause it performs respectably,and is half the price.Quad cores are probably getting cheap since 6 cores are thee rage/.