Yes that PSU is a reliable and great quality piece of hardware. It is quite a bit more power than you need for your setup but you can go two directions with that:
PSU's achieve their highest efficiency around ~50% load. Higher and lower power draw results in lower efficiency. So around that ~50% load, your computer is using it's least amount of power to heat generation/work done in the PSU.
The power draw of your system if you added in a reasonably high end GPU like a GTX 760 or 7950/280/280X would be right around that 300 watts max while playing some demanding games.
The other school of thought on getting a PSU that supplies a lot more power than required is to get a more quiet system. The lower the load of your computer compared to the max power of the PSU, the less the fan in the PSU will need to spin, if at all in the case of hybrid PSU's like Seasonics or other silence oriented PSU's.
Either way you look at it, that PSU may be more than you currently need but it sure will give you flexibility for a high end graphics card while keeping your system quiet and efficient.