Thanks for all the advice!
Problem solved: it didn't use the correct connectors. The card indeed came with two PCI-e to molex connectors. I assumed that this meant they should be used. But the only molex connector available was the one at the end of the power bus supplying the HDs and DVDRW. After re-examining, I found that my PSU already has 2 6-pins connectors needed to power the graphics card.
So no need to buy a new PSU... being ignorant and being cheap is definitely a losing combination

.
Anyway,thx again for taking time to answer what must be pretty noob questions...
Cheers,
Moss