Using only 2 boards has a number of benefits:
- The cards take up less space in the case, leaving more room to route coolant tubes, making it easier to work on and improving airflow.
- 4 HD 5970 GPU's generate less heat than 4 GTX 480's, making it easier to keep them cool.
- Only 2 PCIe x16 slots are required, rather than 4, making it easier to find a compatible motherboard. (Boards with 4 PCIe x16 slots capable of x16/x16/x16/x16 are uncommon, as the Intel X58 chipset does not support than many PCIe lanes without help from at least 2 nVidia nForce 200 chips. I have yet to find a motherboard that has 4 full-speed PCIe slots and supports both SLI and CrossfireX.)