First off, I doubt that you overshot your abilities as the basic premise on building a computer is plug stuff where ever it will fit (well at least in my opinion).
Pentium 4's a lot of power supplies and so likely your 300 watt supply is just not enough.
If that's not it, try using another motherboard as PC Chips is not that great of a manufacturer and bad motherboards often are the cause of computer problems. Also, I can't find your specific model of motherboard on their
site for P4 motherboards.
Also, if you would like I can give you instructions step by step in case the instructions you are using have an error in them somewhere.