hi blue dragon,
in general, software overclocking is useless and will not let you gain any significant overclock. your athlon-XP's multiplier is locked, so you can only overclock by raising the FSB (unless it's an XP-M).
a software overclock will not work because you cannot lock the PCI/AGP bus through software (that I know at least) and PCI/AGP cards are very intolerant of bus overclocking. another reason is that you cannot choose a memory divider to run the RAM near it's rated speed (the only way around this is to buy faster RAM that can handle the overclocked FSB)
if your BIOS has overclocking settings (i.e. FSB, multiplier, VCORE, etc) then overclock through that.
the wattage doesn't matter. overclocking requires stable and sufficient power (not just more wattage). if you want to upgrade your PSU, then a 350w should be more than enough, but don't just buy some generic brand... make sure that it's from a reputable brand like Antec, enermax, or fortron to name a few.
cheers :wave: