i don't think you will really notice a difference in normal windows operations/browsing as the 635 @ stock will be more than enough to handle those tasks. You will see about a 7 - 13% increase in FPS for games though, especially with a decent overclock.
With the H50 you should be able to hit moderate - high overclock on that CPU.
if going from stock (2.9) to lest say 3.3 or 3.4 you may be able to OC without upping the voltage but anything higher and you will have to increase the voltage in small amounts to see what is stable.
Remember, every motherboard and CPU is created different, so lets say i have a 635 and i can OC it to 3.8, using the same motherboard, yours might only make it to 3.6... that is just how it goes. Same holds true for motherboards, some motherboards allow for higher FSB's than others...