You can use two molex to 8 pin PCIe adapter like this one [IMAGE]The Apevia psu has three 4 pin molex which is raw 12V power you can use for video card. [IMAGE]     PS optiplex 750 has a slow cpu which will bottleneck RX570. Apevia is not a good name brand psu.  better consider good name brand such as EVGA.  
Hello Forum: My son is trying to upgrade his video card (XFX Radeon RX 570 RS XXX Edition 1286MHz, 8gb GDDR5) on a Dell Optiplex 750, with an Apevia power supply (Apevia Raptor ATX-RA450W 450W ATX Form Factor Power Supply).   Does any one know, what kind of cable adaptor I need to connect the 8pin video card to the Apevia Power supply? Thanks ahead
Need cable adapter for 8 pin video card