Sure your molex adapter isnt faulty. If the molexs work themselves, stands to reason, they should run through an adapter. Try diff molexs?
In anycase, realize the GT 280 calls for minimum 550W, personally I'd want straight draw, and would get one with a 8-pin, but thats me.
Penewah!
Minimum of a 550 Watt power supply.
(Minimum recommended power supply with +12 Volt current rating of 40 Amp Amps.)
MSI K9A2 790FX / AMD Athlon 64 X2 6000+ Windsor 3.0GHz/Xigmatek 120/ Antec 900/HD4850 Xfired/OCZ Platinum Rev2 2 x 1GB DDR2800 /Samsung Spinpoint 320GB SATA/ PCPC 750W CF Silencer/ LITE-ON 20X DVD±R /WD My Book 500GB/ Also Intel E2180@2.88 Ghz/GIGABYTE GA-P35/CoolMast Case Centurian/Biostar 780G/AMD 4600+ x 2 @ 3.0/Logysis Acrylic/Vista/Ubuntu 8.1 / Corsair 550W PSU