Final answer:
CrossFire is the technology that allows multiple video cards to work together in a single system, enhancing graphics processing capabilities.
Step-by-step explanation:
The technology that allows multiple video cards to work together in one system is known as CrossFire. This technology was developed by AMD (Advanced Micro Devices) and enables users to increase their computer's graphics processing capability by using two or more compatible graphics cards in tandem. The other options provided, Nehalem, Firewire, and eSATA, are unrelated to the multi-GPU setup. Nehalem is a microarchitecture for CPUs developed by Intel, Firewire is a serial bus interface standard for high-speed communications and isochronous real-time data transfer, and eSATA stands for External Serial Advanced Technology Attachment, used for external hard drives and other storage devices.