My PSU is an OCZ StealthXStream 600w. Everything is plugged in and running but I've noticed that sometimes when I'm in a game, my monitor will suddenly lose any signal and just go black, but my PC will stay on. I'm worried that it's my PSU not being powerful enough to power all the components.
Also, I have 5 pre-built fans running in my case, as well as an Artic Freezer 7 CPU cooler. I've also overclocked my graphics card to 900mhz/1300mhz. I have no artifacts whilst playing games, though.
I always thought 600w would be enough for what I've got, and the brand is reliable, too. Does anyone have any advice?
A decent 500w PSU is more than enough for that rig, your 600w would have no problems. It sounds like an overclocking issue. You may not be seeing artifacts...but that isn't the only thing that says an overclock is good. See if it keeps happening with a lowered OC or even stock clocks. It could just be that you need to increase the power to the GPU.
+1 to Trapper:
OC or driver issue is a lot more likely. Check your system event log to see what error code you're getting in the lock-ups. Does it lock up during stress tests (Furmark and/or Prime95)? All games or just certain games?