Yes, you can overclock your gpu.
Yes, it would be in your best interest to get a better cpu cooler.
here is a link to a cooler I recommend(make sure it will fit in your case)http://www.newegg.com/Product/Product.aspx?Item=N82E16835103055
You may want to even look into a liquid cooling system
Your PC should have no problem running two gpus
Finally, you should consult the overclocking section of this site. they will get you set off in the right direction. there are also many guides around the internet that you can use as resources.
I hope this helped some and best of luck overclocking your pc