For Search, How often should you perform an incremental crawl? I was thinking of doing it every two hours? Is that too often? Are there any best practices in terms of frequency of searching?
It depends on how frequently you want search to include recently added content.If the content on any site is updated too frequently (like Sharepoint.StackExchange :-) ) and you want user to search it out, Set high frequency.
However, Remember that Crawling happens at the cost of performance.
Please check this article as well how to plan crawl : http://technet.microsoft.com/en-us/library/cc262794(office.12).aspx