The difficult question is Core 2 Duo or Core 2 Quad. Most people would say "go for Duo as they are cheaper and most programs dont take advantage of quad cores yet so why bother?". If you arent going to run any heavy programs then I'd go for a Core 2 Duo, then buy a quad if you find you need it in the future when they become cheap (And obsolete?)
But then some people argue "buy a Core 2 Quad now, the price difference isnt too bad. Why restrict yourself now, you might want to play an awesome game/run a hungry program tomorrow but can't because your CPU is slow".