Keb ook wel ergens gelezen dat er nogal veel energie verloren gaat bij meerdere (toch meestal meer dan twee) rails, elke onderdeel krijgt dan een aparte rail toegewezen en zo is het mogelijk dat er niet genoeg stroom is voor bvb de CPU terwijl er stroom op overschot is voor de GPU, dit omdat het niet mogelijk is voor 1 rail om zijn stroom te verdelen over 2 toestellen
bron: Sapphire Support forums, Troubleshooting, Sticky: Read this first
NEW ADDITION - Single-rail PSU recommended for high-end systems.
As suspected for a while, there finally seems to be some conclusive evidence, that single-rail PSUs are infact better for power-hungry high-end systems, than their multi-rail counterparts.
The correct phrasing would be: Any PSU is good enough if any of its 12V rails delivers enough power to drive the processor AND the video-card at the same time.
That usually means at least 22A per rail.
If you're using an X1900XT (let alone two of them in CF ) you should have a 22A+ continuous output rated 12V rail driving it (them). While you should be OK if there are two or more such rails (which DO NOT share their outputs) available, you probably won't be OK even if there are five 18A 12V rails in your PSU. (PSUs that have the Split/Combined 12V rail function are an obvious plus here.)
This issue was suspected by many for different reasons and one of them is: it is much more expensive to make ONE strong 12V rail then it is to make many weaker ones.
Source:-
http://www.theinquirer.net/?article=29940
Update on the matter:
http://www.theinquirer.net/?article=30053
Bottom line: PSU designs cannot keep up with the current high-end rig power requirements. When building a new system, keep a healthy (50%-70%) upwards margin for a PSU.