If you intend to build a system today with a high speed Athlon or Pentium 4, 512 MB of DDR(2) RAM or more, a big HDD and a top notch Radeon or GeForce, then a 300W won't even come close. You need at least a 460W of an A-brand (like Enermax) or even higher.
The processor alone will dissipate anywhere between 80 and 110W, and the same goes for your video chip. Thats around 200W for only two chips. Then there's the north bridge of the chipset (you don't think that big cooling block is just for looks, do you
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), something like 50W for the RAM chips, 40-50W for the HDD drive (even more when it's spinning up, that's why server PC's are spinning HDD's up one at a time), and I haven't even touched additional stuff like sound boards, CD or DVD drives, burners, etc.
And keep in mind that not all the W's coming into the PSU will come out usefull either. Even with a top brand, you still loose something like 10-20% to heat dissipation.
I'll leave the final calculations to yourselfs, but if you can add ....
Jan Peter