I've heard great reviews about the 555 cpu and I have seen people unlocking the 2 cores and then overclocking that to 4.2 Ghz. What motherboards are really good at unlocking cores/overclocking for a fair price? ($cheap as possible to $110~)
thats a very good build. even a dual core can handle most multitasking (especially simple stuff like office/browser/music)
a more powerful dual core is more beneficial than a less powerful quad right now, because quads aren't that well supported. you can't rely on unlocking, but the sucess rates are high, and if you are successful you will have gotten a bargain.