Hyper 212 evo and gtx 670 fan

Stock Evo? What stock evo? The evo is nice as it is. Also, most GPUs should ship with their own fans.

No i mean replacing the stock fans of both with something better

There is ONE, but it is unnecessary.

I seriously don't know why you would replace them... both are very good. What's your budget?

nvm then

You would have a very difficult time just replacing the fan on the gpu.

Set the fan speed to 60% and you should be fine while gaming   (use MSI Afterburner or EVGA Percision X)

Replacing the evo fans is very easy and only requires new fans and the old mounting brackets/extra that came with the cooler. Fans should be 120mm.

Check out Frozencpu.com / Xoxide.com / Newegg.com / Amazon.com for fans  

Aerocool/Scythe/Gelid/Phobya/Silenx/Noiseblocker all make excellent fans with great dba/cfm and colored/leds.

Would using a aftermarket cooler lower my cpu temp by much with overclocking?

Yes. Higher-grade coolers do help with CPU temps. However, the EVO should be enough to keep temps pretty low. I have ONE fan and it keeps my idle temps well under 40C when OCed to 4.5 GHz and my load temps well under 60C. Btw, you can only use CM 120mm fans on the EVO.

Edit: Typo.

Pretty much this.

If I were to upgrade the fans on a cooler, I would invest in a better cooler before spending the money on new fans and a cheaper cooler.

$20 for fans + $30 cooler = meh.

$20 for fans + $50 cooler = better  

$20 for fans + $70 cooler = excellent  

alright, ill just keep the stock fan with the evo