The RV770 is perhaps the best thing that happened to AMD in a long while. But more than AMD, it's perhaps the best thing that happened to us, the consumers. The new PCB's designed by ATI for RV770 cards are actually running at well below the clock speed they can support and there is every reason to believe that these cards will be challenging NVIDIA's very best.
The HD4870 PCB with two 6-pin power connectors can support a maximum TDP of 225W (2x 75W from the power connectors + 75W from the PCI-Express interface). While at stock parameters, the HD4870 will not consume over 170W, it implies that with a fair bit of binning for high-performing parts, there is a serious lot of room for overclocking way beyond what the ordinary HD4870 cards can take.