evga 450 watt

I have a dell optiplex desktop with a core i5 2400 CPU, 8 gigs of ddr3 ram, and a HDD 250 gb. I don't have a graphics card yet, and on craigslist I saw someone selling a Rx 580 4gb readeon gpu, for 80 dollars. Which seems like a good deal, but most say it requires a 500 watt psu. My psu isn’t...