What accessories are you wanting to add? Fans don't use a lot of power. We're talking less than 5 watts. My initial response is you don't need to upgrade. You surely don't need to buy a 700w PSU. If you were to upgrade because you wanted a better PSU in general, a 500w PSU would be just fine.
It's not always about watts. Efficiency and quality are more important that wattage.