The only thing you might have issues with later on is the video card. For most games out now it will work fine, but I can see a time in the not too distant future when it may not have enough power to game. If this isn't a problem with you, then the system should easily last your desired 2-3 years.
As for quad core, either that or a good dual core will work well. Not a lot of programs are really taking advantage of four cores, but odds are good that this will change. Something else that will likely change is base operating systems. While XP and Vista both offer 32 and 64 bit systems, I think that 64 bit will be the norm before too long.