The HD4850 is a single-slot card and has a cooler very similar to it's predecessor, the HD3850. The HD4850, with lower clock speeds and GDDR3 in stead of GDDR5, consumes less energy than it's big brother. 1 75 watt PCI-Express connector is enough.
The HD4870 has a big dual-slot cooler and requires two standard 6-pin PCI-Express power-connectors. It's consumes more than it's predecessor, the 3870, but it requires less than the GTX 280 which needs a 6-pin and a 8-pin PCI-Express power-connector
AMD suggests a 500w for the 4870 and a less 450w for the 4850
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