I highly doubt you need that much power... I think you just need a quality power supply. People have gone as far as to call mle a liar when I tell them what I'm running and what PSU I'm using. But here it goes anyway...
Antec 350 watt (the one that came in the Performance II Workstation Tower)
A7N8X Deluxe w/onboard sound and NIC
XP2500 @ 2.3 Ghz on 1.725 volts
1 GB of PC3200 RAM, (2x512), 2.8 volts
WD Raptor hard drive
WD800JB 7200 RPM hard drive
DVD-ROM
CDRW
3.5 floppy
6 in 1 media card reader
USB optical mouse
GeForceFX5900 @ 490/950
2 2500 RPM 80mm front intake fans
1 2500 RPM 80mm rear exhaust fan
1 2500 RPM 80mm slot exhaust fan
1 3500 RPM 80mm Panaflo CPU fan
All that is running on an Antec 350 watt power supply... now, you can't tell me, and expect me to believe that your power requirements are so much greater that you need more than 420 watts. I could add a couple more hard drives and some stupid neon lights and still not need 420+ watts.
List all your componants so we can help you decide on something... cause 550 will be overkill unless you're running dual Pentium 4's or a RAID array with 8 hard drives or somethin crazy like that.