Extra fans will never hurt. And as to how much of a drain it will be on your system, well... that depends on the fan. If you look around, you'll be able to see what kind of wattage it will draw from the PSU. To start you off in your research, here is the address to Delta: http://www.deltaww.com/
As for your current PSU, if you start to have issues, then replace it. Some 300 watt units are built very well and can take the extra workload. Others can't.
Good luck. :)