Power Supply

Hi have a rather new pc (Compaq-almost a year old now)...I want to update it..but I really cant untill I get a larger power supply, this will be my first time getting one so I ask yuo this.....my current power supply says its an ATX-250watt-12Z REV.: D Input:100-127V-6A, 200-240-3A 50/60Hz BST ATX-250-12Z D...so I asume getting another power supply that is "atx" and amd compat it will work on my pc?...also I dont see measurements on any thing I have looked at..are all power supplies the same size?


Before you say "Compaq..what were you thinking yo uidiot" let me say I was in a rush for a pc I needed it for school and didnt have time to shop around....


SPECS

256MB RAM (yeah yeah gotta get more)
250 WATT PW
AMD ATHLON XP 2400+
2 40GB HDDs
Windows XP Pro
Geforce FX 5200 128MB Ram
1 CD/RW
1 DVD player CD-RW combo