Seems to me like the extra cost is worth it, unless you are shaking the couch cushions out to pay for this project. A quality 850 watt power supply with a single rail will likely last you a lot longer than two of the cheapest 450w units, and you can get one that is 80+ platinum certified for about $130. The efficiency alone would be a significant energy savings in the long run. You might be saving money now, but you will pay for it in the years to come.

http://www.amazon.com/Seasonic-PLATI.../dp/B00608FKN8

As an example, you can buy one of those used for about $120, and at 900w load they show a 90 to 91% efficiency. This is phenomenal.

Meanwhile, cheap off brand units with no ratings on them can dip down to 50 or 60% efficiency when you run them at their peak.