The Phenom II X4 is good, but get the retail box, not the OEM. You get HSF, and I think a longer warranty.
And you don't need to re-install windows, but when I went from Sempron to Phenom II X3, I got blue screens 'til I re-installed, although the SECOND time I went from Sempron to Phenom II X3 (Had to downgrade temporarily), everything worked fine.