You may want to install a new power supply. One thing to look for is whether the size of the mounting area for the power supply is large enough for a power supply with sufficient capacity. You can get the dimensions of the new power supply from the listing on the vendor's site.
Second, Look on your motherboard for a 4-in auxiliary power feed, either at the end of the main power socket or off to the side. The newer power supplies have an auxiliary feed that powers the PCIx. If you have a PCI express slot, your motherboard should have one.
I recommend you look for a power supply that is quiet. I installed a CoolMax 450W quiet power supply to replace a noisy one, and the difference is worth the $40 to $50 cost (in my mind - noise level is a source of fatigue).