I5 750 CPU cooler?

you mean noctua NH-D14  

Best answer selected by yogman.

THANKS!

but out of all of them, which one would you think would be best?

Thanks for selecting me!
The Prolimatechs and regular Venomous X require you to purchase fans.
The Venomous X-RT comes with a fan.
The Noctua comes with 2 fans but the unit is huge.
Those 4 units are generally considered the best performing within 1C of each other.
What you have to consider is cost and if the unit will
1. fits in your case
2. doesn't intrude on your dimms
3. your ram height doesn't interfere with installation
The CM Hyper 212+ is a smaller unit that performs within 6C of those said units and
it will present less of an installation problem if spacing is a concern.
It's a very good unit that cost less then half of the others.
Since you have an i5 as opposed to an i7 9xx series the amount of heat created when oc'd is much less.

Also, for achieving the 4Ghz mark, is air or liquid better?

Obviously, liquid would be better, but you don't need it. I've got the i5 750 clocked at 4.00 GHz with the Cooler Master Hyper 212 (Only $30 USD on Newegg.com) with just the one fan that came with the cooler. My temps don't get up above 72c. 100% stable too.

In my opinion, anything better than the hyper 212 is a bit of overkill if you just want to hit 4.00 Ghz. But if you DO want to go above that, or get better temps, I'd go with the Prolimatechs Megahelms.

Thank you SOOO much for all of your help!!!!

Also what case do you have?