Get a "K" tagged processor. Considering you''ll have it for the next 2-5 years, you may as well have the overclocking ability at your disposal. It may not seem important at the moment, but a couple years down the road, when you're looking for a smidge more performance, you'll have it.
Granted you'll probably see no difference in the real world from one chip to the next, even overclocked on any given platform. It's nice to have the option.
By the time any given CPU is due for an upgrade, it's time for a new mobo (chipset), RAM type (DDR2>3>4), anyhow.
Get a K and use it for the next couple years, then overclock for the couple after that, then new PC.