Your PSU cannot handle that load. Not even close. The PSU would be lucky to handle two GPUs, much less two GPUs with 7 HDDs.
Also, unless you're board states that it supports three-way SLI (or just SLI for that matter), you should assume it doesn't. If you can't use the extra cards, the upgrade certainly isn't worth it. To be honest, I would just go out and grab a single HD 5770 for about $150.