Oh good, another slimline. I wish people would stop buying those.
The absolute best thing you can do is take everything out of the crappy slimline case and put it into a regular case. Then upgrade the power supply and video card. Because as others have said, your odds of finding a slimline power supply that can handle anything decent are about zero. When all this is over, smash the old slimline case with a hammer so no one else can use it.
Of course, you could still be screwed if your motherboard has an oddball proprietary power connector. Then your only option is to get a whole new machine. So check that it has a standard 24-pin or 20-pin connection before you do anything. Dell or HP or whoever doesn't make any money if you upgrade your machine with off-the-shelf parts, so they will do their best to make it difficult and steer you into buying a new Dell or HP or whatever. If your motherboard does turn out to be the proprietary kind, don't reward that type of bullshit; just learn to build a new machine yourself instead of staying beholden to the pre-built manufacturers.