<i>Only</i> a 300 watt power supply? Are you kidding? The only things with <i>more</i> than a 300 watt powersupply are rich people and servers. A 300 watt power supply is perfect, and it has a lot of power to spare for tons of other things - DVD, CD-RW, powerful video card, hard drives, etc.
I've been aware of the 400w recommendation for 1.1,1.2ghz athlons, but I think you can probably get away with a 300w. Antec produces exceptional power supplies. Anandtech did a athlon PS review a while back. They found that an Antec 230W was extremely good, better than the majority of 300w supplies out there. In theory you can just read the power rail ratings, but as in the case of the antec 230w, this doesn't seem to tell the whole story.