What you didn't mention is who made the power supply. I have had good luck with high quality supplies running setups that they were way underrated for. I just retired a 235W Sparkle that was running an Athlon XP2400 512MB ram,2x80GB, CDRW and a ATI Radeon 9000. All rails ran within spec. I finally upgraded a little while ago when a client went to a 500W so I got his old 400W Fortron. Didn't make a difference in my system but its nice to have the extra capacity. I've been installing 300W Sparkles in client systems for the last few months and have had no failures out of 7 installs. A couple of these are what I would call "challenging" configs. Not a huge sample but if there was some limitation on the capacity I would have seen it already.
I for one would trust a high quality, low rated PS (like Sparkle/Fortron, Antec, Delta, PC Power&Cooling) over a high watt generic. The fact that you are adding 6 HDDs to your system may tax any low-rated PS so you may want to upgrade. If you do, don't get carried away by the wattage ratings. A high quality 400W will run better than a no-name 500W.