By any chance, is this PSU from an HP computer? My PC used to be an HP, and it had that exact PSU. It was running a P4 540, 1GB RAM, one HDD, two optical drives, an X600 and a SoundBlaster. The CPU and mobo fried after a couple months, but I don't think that the PSU caused it. More likely the poorly designed case. But enough bitching about HP.
13A on the 12V rail is not enough for an X800XL; it might work, but you risk overloading and blowing the PSU. If the PSU blows, you do risk damage to the other hardware.
If your computer is an HP, I recommend getting a new case, along with a quality power supply with at least 20A on the 12V rail. The 12V rail is what's powering the CPU, hard drive, graphics card, and some other stuff, so it's important to have a lot of continous, steady power on the 12V. Also, look for at least a 400W PSU.