If it works, my guess is it's ok, as long as it's not stressing the PSU to failure, and I don't know how to determine that.
If you are worried, by all means update the PSU, if not, enjoy your new video card


Mainboard: Asus P5K-Premium, CPU=Intel E6850 @ x8x450fsb 3.6ghz, RAM: 4gb PC8500 Team Dark, Video: NV8800GT, HDD: 2x1Tb Samsung F3 RAID-0 + 1Tb F3, PSU: Antec 550 Basiq, OS: Win7x64, Display: 24&