The PSU is the heart of any PC and should be the first thing on anybody's shopping list when it comes to a new PC or rebuilding /updating an existing one.
That said, not everybody realises this until it is too late and they have spent all of their money on other components. I know, I've been there and done that.
It is always easy to spend other people's money and sometimes other, more affordable options are required until the finances are in place to purchase the next essential part.
If the existing 650 Watt PSU is still functioning OK then using the 4 pin connector, as I stated before, should supply enough power for this system. If it doesn't then buying the 8 pin cable adaptor is the next best option if finances are tight.
If money is no object then of course, a good quality PSU is the perfect solution. It should last for several years and offer the necessary power cables for future expansion requirements.