Within the past week my computer has been randomly shutting itself off, as if someone just yanked out the power cord. So far the only times I've seen this happen are when I've been playing games. Monitor and speakers still have power, and the light on the back of the PC's PSU is still lit up, so the power/surge strip looks like its still working (and the circuit breaker on it isn't tripping). And the power cord is plugged in nice and tight on both sides.
I've recently added a SoundBlaster X-Fi and a second hard drive in the past month, so I'm starting to wonder if I'm on the edge of overloading my PSU. My system has:
Athlon64 4000+ CPU
2GB dual channel RAM
Radeon X1900XT GPU
2 SATA Hard Disks (160GB and 80GB)
3.5" Floppy and 7-in-1 memory card reader
2 case fans
500 Watt Fortron PSU
The power-offs happen often enough to be annoying, but not often enough to easily troubleshoot. Sometimes I can go several days without it happening, so its hard to pinpoint whether a change I made is helping or not.
Yes, I would say you might be overloading your PSU. You can't rely on the 500W rating. That is a maximum burst output. Actual sustainable output on that PSU is more like 440W. That X1900XT alone requires 9-10 Amps on the 12V. Hard drives and DVD-RW drives are about 2 Amps each. Not sure on the X-Fi, but I would guess that it needs more power than a low-cost soundblaster.
So yeah, looking at the 500W FSP PSU on newegg, you're pretty much maxing out the 12V rail when gaming. 12V1 has 15A, 12V2 has 16A. That's kinda low for a system built around the power hungry X1900 series cards.
Oh and if you're overclocking anything, that's only making it worse.
I also wonder if temperature might have something to do with this. The new Harddrive probably added heat and the X-FI sound card was probably right next to the ATI video card fan (which is know to run hot anyway). Everybody else is probably right that it is the PSU. If that doesn't work, however, you might want to try a different arrangement with the sound card and harddrives to improve airflow.