The HD 4870 X2 has a TDP of 300W. PCI-E slot = 75W, PCI-E 6 pin connector = 75W. PCI-E 8 pin connector = 150W. Although that is with a wide safety margin. The card is never likely to consume anywhere near that much.
AMD Phenom II X4 955 BE @ 4.0GHz + TU120E lapped - MSI 770-CD45 AM3 - 2x2GB OCZ DDR3 1333MHz - Sapphire HD 4870 - Samsung Spinpoint 500GB 7200RPM 16MB cache HDD - Tagan TG600-BZ Piperock - (Currently open test bed) - Windows Vista Home Premium 64bit.