[BUILDHELP]Build whole new system or just replace parts?

Recently I have been having some computer troubles. First there was no display onto any monitor that I had, so my guess was motherboard fried. Then the other day I tried turning it on and nothing happened at all, so I am also guessing that my power supply also went. I would also like to upgrade my video card.

The reason I am wondering is because it has 8GB of RAM and a 1TB haddrive that I don't want to replace. So what would be the best decision, replace motherboard, power supply, and video card...or just make a whole new machine.

SPECS: AMD Phenom Quad Core 2.5Ghz Mobo (Dead?) AMD780G RAM 8GB DDR2 1TB HD Video: AMD Radeon 4650

I already have a mouse, display, keyboard. And I could use the old case if needed and just gut it.

Anyone have any advice?