Be careful with the adapters. If your PSU has twin 4plugs, those are -usually- for the motherboard since some boards take an 8pin mobo connector - twin 4's are usually for that.
If you're running a basic system (mobo, 1 CPU (non-oc), 1 drive, 1GPU), you could use molex -> PCIe adapters to power the GPU if you have a 400w or so PSU. An 8pin connector is rated for 150w of power, so add 100w more for the CPU and 75w for the rest of the system and you should be covered.