Hi. Yes you should get a new PSU, you could end up damaging your GPU and i wouldn't be able to run at full force.
When picking PSU you want to look for what certification it has. Bronze, siler or gold. Gold i the best option of course but it's not necessary. These "rankings" is the PSU's power efficiency. You should at get Bronze minimum. For your "situation i would recommend 650W. I have "silver" myself, and it works very nice. For an example: Silver is also know as "80 Plus" meaning 80 or more % power efficiency. If you buy a 500W power supply and it's a, say 50%, then only 50% or 250W would come to use.
And one more thing: Buy modular!