Black screen after upgrading graphic card - Inspiron 620

motherboard chipset: H61

I've upgraded this machine before: PSU 380W and nVIDIA GTS 450

It works fine for a long time.

Today I upgraded my other PC and got a spare PSU 500W and AMD 7950

So I just replaced the PSU and GPU in my old Inspiron 620

When I turned on this computer, nothing showed up, just pure black screen without signal, and there's no beep on this motherboard, so no alert of course. Then I pushed the power button, it turned off immediately which means it didn't pass POST at all.

Now I'm using it with 500w PSU and GTS 450, works fine.

It's clear that the 500w PSU (12v rail 41A) is enough for this system, both PSU and 7950 works fine in my other PC with a more powerful CPU (TDP 115W). 

So I suspect it's the compatible problem between H61 motherboard of 620 and AMD Radeon HD 7950

The BIOS version is A10. It's the newest one released in 2013.

Thanks for your help!

update20160906:

I just got another motherboard GA-B75-D3V, same thing happen again. If I connect both 7950(hdmi) and iGPU(dvi), I can check the bios using iGPU, but can't enter win10 with it. This is the most confusing part, since iGPU works fine, then why can't I enter win 10? It only shows the win 10 logo for 0.5 sec then black out again.

The other PC that can handle this 7950 is X79 + E5-2670, which support and work at PCI-E 3.0 x16

H61 only support PCI-E 2.0 x16. B75 has one PCI-E 3.0 x16, however G630 only support to PCI-E 2.0 x16. I guess maybe this cause the problem.