So the question must be asked, why are you replacing the power supply?
Is it because you upgraded the fully functioning one already in the PC, or because the one you had died?
If it is the latter, and if the one that died is a low quality cheapo unit then good chances are it damaged motherboard and/or gpu when it died.
PSU connections needed: 24pin to motherboard, 4/8pin to top of motherboard (for CPU), connections to GPU, connections to hdd/odd.
With a modular PSU you need to make sure that you are plugging the right cable into the right kind of port on the PSU. Your wiring for a GPU can be quite different then that for sata/ide, and plugging your GPU power cable into a sata power slot on the PSU could damage the GPU.