Time for a new gaming PC...

Today, my gaming PC began to fail. I bought it as a Gateway DX4300. Underwhelmed by the GPU, I upgraded to a XFX Radeon 6790. I also bought an Apevia Java 650 watt power supply to accommodate the beefier graphics card. As of today, my computer will run for around 5 minutes, and shut down. The fan sounds like a B-52 bomber. I think I will replacing everything except for the GPU and the PSU. If you have any ideas on what my parts should be, please tell me in the comments.