300 Watts is cutting it close ... I'd have a 400Watt or better so that the PSU is not having to run at 100% all the time which will lead to it failing sooner than if it was running at say 75% if a 400watt. PSU's also are not that bad in price generally, and if you burn up that 300watt supply it could take out more than just itself, it could damage your motherboard as it fails and can not regulate its voltage outputs correctly. You have to weigh the risk. Can you chance burning up the motherboard or is it smarter to have a stronger PSU that can handle better loads.
The other danger running 100% is that you can corrupt your hard drive as the amperage and voltage available dips and the platters dont quite spin at the correct speed, but slower in RPM's and the data ends up intruding on neighboring sectors as the drive starts to lag behind the data it is writing to it and you can suffer total data loss or a very costly data recovery if that important.