With Web Gateway 7.5.x gaining steam it's important to note that it includes a 64-bit AV engine. With this improvement it is recommended to upgrade the RAM in your Web Gateway.
At a minimum it is recommended to have 8GB of RAM when Gateway Anti-Malware is used.
Minimum requirements for all platforms (virtual and appliance) have been updated in the latest Installation Guide: McAfee KnowledgeBase - Web Gateway 7.5.2 Installation Guide
Please check out our guide on for upgrading the memory: How To: Upgrade the memory on your B Model appliance
Older KB: Please check out the KB listed below about what kind of memory is supported in our B model appliances (4000B, 4500B, 5000B, 5500B). The KB includes specific memory modules which can be purchased.
McAfee KnowledgeBase - Web Gateway 7.5.0 recommended memory (RAM) upgrade