There isn't a whole lot of extra room in the 300, so I doubt that you could fit a 2nd normal sized PSU in it. Even if you could, cable management would be a little insane. But as others have stated you could try a 5.25'' Bay style PSU.
As far as a GTX295, it will fit, but you may have to angle it into the HDD cage before you try and seat it in the PCI-e slot. You may also have to rearrange your HDDs as well depending on how many you have and where you have them placed.