If you have an AMD system over 1GHz, you should have at least 300 watts. If you don't have enough power, you will have many problems, mostly instability where the system will crash when under heavy load, or just booting up.
The power supply to get is dependent on your system. How fast the CPU is, how many cards and drives you plan to have... So what will your system contain?
Turbo Gamer 21" Black Aluminum Case w/ 350watt Enermax Power Supply.
AMD Athlon XP 2500+ 1.83GHz @ 2.09GHz 1.65v (167*12.5)
ATi Radeon 8500LE 128MB DDR
Gigabyte GA-7VAXP KT400 w/sound+LAN+Firewire+RAID
2 x Samsung DDR SDRAM 256MB PC2700
Seagate Barracuda IV 80GB 7200rpm ATA-100
LiteOn Black CDWriter 42x24x42
LiteOn Black DVD-rom 16x
Panasonic Black 1.44MB 3.5"
Windows XP Professional
Creative Inspire 5200 5.1 Black Speakers
80mm LED Case Fans x 5