Well if you have a good psu and the PCI connector is on a rail that can handle it its no problem. Those 3 12v wires can handle the wattage only the PSU should be able to do it. The pci 1.0 specification is rated 75 watt for the connector. So a 1.0 certified connector should at least be able to handle 75 watt. But new psu's can easily do more.

The HD4870 is pulling about 40 watt more then a HD4850. A HD4850 on 1.23v 776-1125 is pulling 5 watt more then a HD4870 on 790-1100. The HD4870 is running 1.28v.

If you go from 1.19v to 1.23v on the HD4850 you get + 25 watt. So you can make a indication of how much 1.45v will use on the HD4850.