A system with a single HD 7870 requires a 500W PSU with at least 28A on the 12V rails and one 75W 6-pin PCIe power connector.
A system with two HD 7870s running in two-way Crossfire mode requires a 600W PSU with at least 40A on the 12V rails and two 75W 6-pin PCIe power connectors.
There are three models of the Thermaltake Lite power 600W. The LT-600 and LT-600P are made by Solytech and the W0356 is made by HEC. All three have at least 40A on the 12v rails, sufficient PCIe power connectors and technically meet the minimum requirements for an HD 7870 Crossfire setup. I consider your current PSU to be decent...at best. I do recommend upgrading to a good quality PSU, but you can stick with a good-quality 600-650W PSU. Something like the Seasonic S12II-620: http://www.newegg.com/Product/Product.aspx?Item=N82E16817151096&Tpk=s12ii%20620
or the Corsair TX650V2: http://www.newegg.com/Product/Product.aspx?Item=N82E16817139020&Tpk=tx650
are both sufficient for your build and great quality PSUs. If you're not purchasing in the US then give us a site to look on and help you select a PSU.