Overloading a PSU or loading it to 100% shortens it's lifespan and depending on the quality of the PSU can cause hardware failures. Don't get cheap on your PSU.
On the more technical side a brief search on Tom's showed this video card will eat 163 watts of power under load. I wouldn't run a PSU beyond 80-90% of its rated capacity which on a 450 watt PS leaves 360-405 watts. So this video card at load with a 450 watt PSU is taking up between 40-45% of a safe load by itself.