I remember an EVGA GTX 660 requiring a 450w PSU and it has a 140w TDP, about 5w less than the GTX 970. My system running a single GTX 970 draws up to about 400w max without overclocking. You could get by with an upper-tier 450w or greater PSU on a non-overclocked PC. If you have a 400w PSU, then I would advise you to upgrade it. If you can find a good sale on a Tier 2 500w+ PSU, then by all means go for it. I see XFX TS PSUs on sale on Newegg right now.