New Video card won't boot

Hi, I recently change my Sapphire Radeon HD6950 for nVidia MSI GTX-1060GT OC, the new video card works perfect!!!

So I give my old Radeon to my Little brother** (he also play PoE)... the HD6950 will double the preformance of this old GTS-450, the thing is the PC won't even boot whit radeon placed... On board video works fine (Core I5-3470), the old GTS-450 works fine.

Only diference it's HD6950 uses 2x6 PCIe power pins and GTS only one 6 pin.
PSU it´s new, bought yesterday (Thermaltake TR2-600W) and looks fine to me, it's not the best PSU over there but should be fine for that.
Another difference is PCIe 2.1 (Radeon) PCIe 2.0 (GTS) and the mother is Intel Q77 (I don´t know what PCIe have).

Is not drivers, because don't even boot, is not the card because still working in my PC, PSU? I doubt it, but who knows?

When I turn on the system nothing happens not even one beep, black screen, all fans works, even GPU ones, HDD works fine too.

It's like there is no video signal, PC may be starting normally, but hey can´t tell without image in the screen.

Right now he´s using the GTS-450, but I like to know why the Radeon don´t give signal in that PC, when works perfect in my machine.

**My little brother is 25 years old, but he always will be my little bro.