It seems that your power supply is good enough. I'm guessing you had your computer custom made, so maybe you should check an online website that can help you check how much voltage your system consumes right now, and see if you have atleast 100w after you add the second gc. The 100w is just an extra safety precaution, to make sure you dont overdo it and end up frying your system.
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Hope I helped you,
Harchint Arora
www.harchintosh.com
PS: If you could help a fellow computer geek out, it would be awesome if you could take a minute to just visit my website and click on an ad, wait 10 seconds, and maybe even check out my site. This would help me pay for the website as i'm a little strapped this month. That would be really awesome if you could man, thanks!