What else you got in that P3 550 tower that you need so much juice?
Have you calculated your rail requirements yet? Opus 150W may be more than enough to power your setup.
I power a P4 2.4 system off the Opus. I've seen a few posts here of users running AMD Athlons off the Opus. Both use significantly more power than a P3 550. Its not quite an apple to apple comparison, as newer CPUs tend to use the 12V rail while the older ones (ie P3) use the 5v rail.
Verify the rail and power requirements. Search google for PSU power calculators.
Heres one that was stickied here in the past:
http://www.mymp3car.com/MP3Car/PSUpower.asp
The CPU is not included the rail totals for that particular calculator. You'll need to do the conversion yourself (P = IV or I = P/V using the right rail for your CPU).
Once you know what you need on each rail, compare it to the Opus data sheet. You can also give Kris at Opus a call. He may have tested a similar setup or have customers that use it on a similar setup.