Without any tools provided by Gigabyte, overclocking the GV-NX78T256V-B was achieved using the Coolbits registry tweak. We went into the driver settings, and raised the core and memory clock speeds until we began to see artifacts or experienced other instability. At that point, we moved back a few MHz until we had a completely artifact-free environment. Our final results: we moved the GPU clock up to 473 MHz, and the memory to 565MHz (1130MHz effective). To see what kinds of gains this achieved, we ran two benchmarks a second time using one synthetic test and one real-world game.
In the end, we wound up making a fast card faster. The 7800GT had already ruled much of the testing, but raising the core and memory speeds as far as we did allowed us to get an additional 10-15% boost in performance. Keeping with the "keep prices low" mentality that we started out with, we wanted to point out that at 473MHz/1130MHz speeds, the GV-NX78T256V-B is running with higher clocks than a typical 7800 GTX.