3891 psu spec is either 260w or 360w.  If you current psu is 260w, it has no 6 pin PCIe connector.  If it is 360w, it has one 6 pin PCIe (black) connector.There are multiple Dell proprietary 360w to 500w psu that can serve as upgrade of your 3891 psu upgrade.  All these use Dell proprietary white 6 pin connector.  Dell has been using a ubiquitous proprietary 6 pin 12V only connector on almost all of its recent desktop model motherboards across the board (Inspiron, Vostro, XPS, optiplex).  All these psu are interchangeable across models.  The 460/500w psu have two 6 pin PCIe connector (black). Dell makes 500w, 460w and 360w psu in the Dell Thin Form Factor (TFX) that will fit 3891 chassis. 2VD0G 360w Liteon 19YR3 360w 5K7J8 500w Delta99TPH 500w LiteonY7R0X 500w Chicony 4FWF7 460W Chicony HG9XD 460W, Acbel 92WNC 460W, Liteon PS In the very latest Aurora R12/13 and XPS8950 Dell has advanced from the 6 pin to standard 10 pin ATX12VO.
I recently got a dell inspiron desktop and i wanted to upgrade the gpu inside of it, but first i need to replace the psu. I currently don't know what psu to buy that will be compatible with my computer Model number- Dell Inspiron 3891      
What psu should I replace in my inspiron