You would probably be seeing signs of instability already if RAM was going to be a problem for your PSU. It doesn't (compared to mechanical devices like fans, HD's, Media drives) use a lot of power.
As for heat, RAM will generally get hotter the faster it's run, but again if your system is fine now (albeit a little warm) then another stick shouldn't push it into a danger-zone.
Basically as long as the RAM is working when it arrives, and is compatible with the existing modules then it's probably the least impact of any upgrade you can get. The most common problems when upgrading RAM are incompatibility or dead modules.