not necessarily, best to dbl check, some of them may assume you use a molex to PCI-E converter
They both use a standard PCI-E connector but NVidias tp range cards tend to require more power and as such 8 pin plus 6 pin connections versus ATIs which tend to only need 2 6 pins
Most PSUs have one 6 pin and one 6 + 2 pin or double or even treble so the difference is unimportant, Id still dbl check though