The calculators are wrong, the HD 7870 calls for a 500w or higher unit with 2 6pin PCI connectors and 23A on the 12v rail.
Aside from that, don't EVER buy a Thermaltake TR2 unit. They're a tier 5 unit and are total trash. That exact unit is what brought me to Tom's Hardware in the first place and I've seen case after case after case that were nearly identical since then. You want a Tier 1 or 2 unit. Even a Tier 3 unit would be better than that.
PSU Tier list:
http://www.tomshardware.co.uk/forum/id-2547993/psu-tier-list.html
My first Tom's thread:
http://www.tomshardware.com/answers/id-2194316/likelihood-psu-failure.html
There are about three hundred, or more, just like that one if you'd care to search them out.
Basically, you want to use the GPU recommendation to determine the necessary capacity, according to one of these two lists:
http://www.realhardtechx.com/index_archivos/Page362.htm
https://forum-en.msi.com/faq/article/power-requirements-for-graphics-cards
And if you plan to overclock either the GPU or the CPU, add 100w to that recommendation. At least, for the fast and simple version. There are more complicated ways of determining the necessary capacity, but since RealHardtechX and MSI have gone through the trouble of providing their recommendations based on extensive testing, it's much simpler to just go by those estimates.