300W is pushing it, especially if you have a PCIe video card. More important than the wattage rating, though, is the amperage on the 12v rail. For a P4-class processor new enough to take on DDR2, I'd recommend NO LESS THAN 20A on the 12V rail, or a total of 25A if it has dual 12V rails.
Ideally, for that system, I'd recommend 450+W and 30+A on the 12V rail, but if you're not going to be pushing your system too hard, a 300W PSU might suffice.
Nearly a decade's experience repairing/building/configuring computers, and educating people in their use.