Watch out newcomer. Members here are so very extremely nice
Technically you can add 8GB RAM to the existing 4GB, but practically there is a very high possibility to get compatibility issues. Unless you're perfectly certain that the extra RAM is identical in specifications, I highly recommend not adding any. As stated before, you probably won't be needing that much RAM unless you're a very heavy user, like on servers or modern virtual machines. Even the heaviest of games, for example, don't need more than 8GB total.
300WATT output is good for your system. If it is a good brand PSU, then it is perfectly fine for these specifications.