@
nostaradu, Were I in your situation, I'd upgrade the power supply and the graphics card. While your PSU's advertised wattage of 650 is
technically enough, the mfg. and model Thermaltake TR2 is on the Tom's Hardware current
un-tiered list. I would swap it out ASAP. I would upgrade it, if for no other reason, but to properly safeguard my build.
Here is what I propose: Proceed with purchasing your new graphics card, but also sell your two old graphic cards. I don't know what vendor your old GPUs are, or if they are precisely modeled 7800 or actually 7850's. For example, if they were Radeon 7850's, you'd get a combined sale price of at least $100 - $150; possibly much more. With that type of money, you could afford an EVGA P2 850: Tier 1 quality, platinum efficiency, 10 year warranty, fully modular, and has enough wattage to future-proof your build up to two GTX 980 Ti's. I also happen to personally own this precise model.
Normally I wouldn't recommend this quality of power supply for your build, but since you have those old cards to sell, you might as well buy the highest quality on the market.
EVGA P2 850
$119.99 after $20.00 rebate card
http://www.newegg.com/Product/Product.aspx?Item=N82E16817438056&ignorebbr=1&cm_re=evga_p2_850-_-17-438-056-_-Product
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PSU tier (quality) list ***
http://www.tomshardware.com/forum/id-2547993/psu-tier-list.html
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power requirements for graphics cards ***
http://www.realhardtechx.com/index_archivos/Page362.htm