Gonna give you a general guide to get you started:
Have a good read through the psu section and look for mention of power calcualtor sites.
For low power try looking at some epia installs - can run off a 90W psu easy
P3 installs (slow ones are possibly able to run off 90W as long as you dont go crazy with fullsize drives but 150 opus probably safer)
if you want more power look for some athlon XP-M installs (mobile athlons) - need more power (at least 150W)