The Zalman 9900LED. I have it right now on my Q9400, and it's at 3.2ghz easy. It's awesome, and no maintenance required.
Freebie version to try it out, on public servers
It's VERY stable with very few bugs. It's constantly being worked on and updated.
Not to mention you get a free companion each week. :awesome:
But in all seriousness, if you want the full-game experience without the price tag, it's the way to go.
Oh, and you get 12x XP. :wacko: