I would go with the i5 4670 or the i5 4690. Since your board cant oc, then you don't need one with a "k". But it would still work.
I would look on the used market for these, such as ebay. You might be surprised and find a i7 4790 or i7 4770 for a similar price. The i7 will have better performance, but when purchased new they are typically 40% more and you wont see a 40% performance increase. However, in the used market you may be able to find one that is much closer in price.