Discuss the roles of data encryption and key management in protecting sensitive information from unauthorized access and ensuring the integrity and confidentiality of data. Respond to the following questions in your post and provide APA formatted references: 

1. What are the key differences between symmetric and asymmetric encryption methods, and in what scenarios would each be most appropriately used?
2. Assess the impact of encryption on system resources and performance. How do organizations balance the need for security with maintaining system efficiency? 
3. How does Public Key Infrastructure (PKI) enhance data security measures within an organization? Provide examples to support your explanation.
4. What are the best practices in key management and why they are critical for maintaining the security of encrypted data?