You need to measure the power you will need (Watts) for powering your system. A simple system like a PII300 with a 6.0 Gb HD and a CD-Rom will need aprox. 40W. You make a choice by how much acessories you will put in your project (LCD screen, DVD, extra HD, TV board...). Also one thing... the Arise power supplys are not ATX, only AT wich means you can only power AT motherboards (Pentium 233MMX at most).