"To go to one's head" means to cloud one's thinking, to cause one to think or behave irrationally or silly or boldly. In your example, it means that investors are eager to invest in anything related to China or the internet without necessarily having any other good reasons. This term is from alcohol "going to your head".
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Indeed.
I'd suggest the original poster to first search this site for idioms:
Search inside for:
box
(in this case, search for head)
and you will get: