WHy not try the retail fan first and see if you have a problem? You will not harm the chip at 850 if the HS/F is properly installed and you don't go nuts on the voltage, if may simply overheat and lock up. If you can load windows but get lockups after some CPU intensive stuff, you may have a heat problem. I'm able to run a 633@950 with the retail HS/F, but my other one needed an Alpha or I would get lockups after 10 minutes of RC5.
Alphas are very hard to find right now, but I think a Golden Orb is a solid product and pretty affordable at $15 or so.
Speaking of RC5, it is a great program to test overclocked stability, why not venture over to the Distributed Computing Forum and take a look? We LOVE new members
. Lots more info available at Paulson's Page