No need to change unless that one is failing, there is way more than enough power to run either of those graphics cards.
I suggest something in the 550W or a bit higher if you find one on sale - but buy one that you've read positive reviews of. I'm partial to PSUs made by Seasonic or Super Flower. Both serve as OEMs for others that put their brands on them, though Seasonic has their own consumer line. But I really don't recommend buying even a Seasonic PSU without having read reviews by qualified sites. The sites I trust are Tomshardware, PC Perspective, Anandtech, and The Tech Report. Jonnyguru also has a good reputation, mostly well deserved. I look for a pristine build quality (no "mostly good" comments in the review) with all Japanese capacitors.
I also consider a warranty of 5 years to be the minimum acceptable, as no modern PSU should last any less than that and I believe the people that put their names on them should stand behind them for at least that long. Some, notably EVGA, offer units with a 10-year warranty. I believe all of their 10-year PSUs are made by Super Flower. (EVGA also offers lesser PSUs that I don't recommend; stick to the 5-year minimum rule.)
So there are name brands that do use Seasonic and Super Flower - this is almost always mentioned in reviews, so that's what to look for. Corsair is an example of a brand that has used Seasonic to make their power supplies.
One major brand that has never used Seasonic or Super Flower to make their PSUs is Silverstone. In my opinion their units are questionable choice because you never know what company built what you are looking at no matter how expensive they are. Their PSUs might get a "recommended" by some site - even one of my favorites - but I've never seen one reviewed that had a pristine build quality, they've always had some sloppiness inside. This may be acceptable to some, but it isn't to me.
As you can tell, my requirements are stringent; others will give you different advice. That's okay, but if you decide to follow mine your chance of coming to grief years later will be very tiny. And by "grief" I mean a PSU burning up and taking out your mainboard and CPU with it - as has happened to me because I bought an expensive power supply from a brand I trusted without reading reviews of it.
If I had read those reviews before I purchased it, well, I would have known not to.