I have an Athlon II X4 645 CPU, 6GB of 1066Mhz DDR3 (Two sticks of 3GB), a CD/DVD drive, and a regular MicroATX motherboard. But I only have a 250 watt PSU. I want to insert an old passively cooled 7300GT from MSI to give my desktop a little performance boost. Will underclocking (Rivatuner) my 7300GT reduce heat and power consumption? I don't have an extra power supply and I don't want to buy one.