My MB has only one 4pin connector close to the CPU and PCIe bus, otherwise I'd have plugged both for sure
The old PSU was plugged by the 24pin connector and this 4 pin cable, and is again now, and everything works.
I made sure the connectors were not bent and cables not otherwise damaged.
The problem occurs with the old CG (GeForce GT 230 only OEM provided, with no external power connector). Not confident enough to test with the brand new GC, and won't until my system boots at the very least...
The new GC I was about to install is a Zotac GTX 275.
I opened a topic and sent a PM to some Corsair gurus, but no luck, and I can't find anything online. Well that's not exact, I digged up some old posts with similar problems, but none included any kind of answer or solution.
Tomorrow I'll bring my PSU at work and try to plug it in a desktop that sits in a corner... If same symptoms, I'll bring back the PSU to my dealer to get a refund/replacement unit.