I'll be honest with you. As a guy I tend to notice a girl on alot of reasons not just one or the other. It really depends on the guy. Every guy likes something different.

Believe me, hormones play the biggest part. We're guys, we fancy girls (I never use that word for some reason). When we reach a certain age, we notice a girl of about our age or older walking across the street, and haven't had a girlfriend in a long time or never had one in the first place our heart starts pounding faster, our breath starts to falter, etc etc. It feels just like an adrenaline rush, but hormones rather get in a guy's way instead of helping a guy out.

Of course, some guys are completely impassive by it. Not me.

Guys do totally not notice a girl when she's smart... I would know

^No you do not know. And your comment alone proves you are not smart.