It all depends on what you have in your system. If you are building a new system, are overclocking, and are having lots of new stuff put in your system then I would recommend at least an Enermax 430 Watt Supply just to be sure and to maintain stability. If not, a 350 Watt Power Supply should be more than enough. You should stay with namebrand power supplies, don't go with generic because they are sometimes not always stable. Enermax and Antec are probably the best in the power supply field.
as far as ur PSU goes.........they do NOT effect the PERFORMANCE of the computer (IE. Benchmarks, etc.)
they will however have an effect on Stability/reliability...ur best off spending a couple bux getting a name brand one...like Enermax or Antec....350 watts is usually enough and id recomend a 350 to everyone unless there running multiple hard drives or even raid array's or anything of the sort......aswell as a high-powered GeForce 4 Video Card and all kinds of PCI cards....a DVD Drive and Burner....than id recomend soemthing better than a 350.....hell a 350 PSU might even be sufficient for that.....its just that every component is different.....
XP 1800 needs about 75 watt
radeon 9000 consumes about 45 watt
cdrw max 30watt
pci cards each max 8watt
fans+keyboard+mouse+mobo max. 50W
harddiscs each 20W.
so a psu with 270W would be enough,too. but don't forget usb ports extra fans and stability. 350W with a good brand would be best choice