How big/fast is the fan on your CPU HSF and Northbridge?
Do you want to put your HDD in an enclosure?
Have you tightened all the screws in everything so there is not vibration noise?
Are the fans speed adjustable?
Replacing the fan on the videocard was a good move, now for the soundproofing material. There is some stuff out there specifically designed for computer applications but you have to buy it from god knows where, and other things work just as well. Eggcrate foam (the foam that looks like an eggcrate) is what I use and works well for everything, just jam it into small spaces and gaps and it absorbs all sorts of sound.
Next, yeah there is a way that you can swap out the PSU fan without killing it. Crack it open (DO NOT FUCKING TOUCH ANYTHING OTHER THAN WHAT I TELL YOU BECAUSE I WON'T BE RESPONSIBLE FOR YOUR DEATH) and unscrew the fan pull it out and pay attention to where its power leads are running. If it is socketed to the curcuit board, then pull it out and put in a new fan with the same socket on the leads (or solder the old socket onto the new fans power leads). If it doesn't have a socket, snip the wires and solder the wires from the new fan into the old ones. Once this is done make sure the fan is correctly oriented so it is exhausting air, then screw it back in and close everything up. Or you could skip this all and buy a new PSU.
Now, you could put an enclosure around your HDD and then it would be silent, they're only about $20 or you could make it yourself, but its a good option.
If you can, it would be a nice thing to replace all the 80MM fans in the case with speed adjustable ones so that you can turn them down and quieten them, or if you can put 120MM fans in instead, do it.
I realize I didn't organize this post at all, but I hope it answers some questions