I agree with you, if ONLY for gaming and basic web browsing, I would go with 4690k, but if you plan on doing any thing more "CPU-intensive" I would then go with the 4790k. If you were going to keep the 2600k, I would prefer that above either the 4690k or 4790k. But you stated it's from scratch, then 4690k it is (IMO). I also think an mATX board costs less than an ITX board...I looked at the Asus Z97M-Plus and their Z97I-Plus and the mATX board is about $30 cheaper. Here is a link for both boards:
Asus Z97M-Plus: http://pcpartpicker.com/part/asus-motherboard-z97mplus
Asus Z97I-Plus: http://pcpartpicker.com/part/asus-motherboard-z97iplus