PC dead after PSU overheats, now only turns on for split second

Hello guys,
I've been reading some helpful posts here, and tried to follow the troubleshooting list posted.
Unfortunately my Mobo's speaker's wiring is off (loose) and it won't beep, even when i hold it together.

My main problem right now is that the computer will not turn on when the 4 pin power cable is connected to the mobo.
(it will light up for a split second then dies).
When its just the 12pin power, it can turn on (fan lights up and spins), but monitor etc still off.
A
First of all. Specs:
CPU: AMD Athlon Phenom 2 X6 1090
Mobo: MSI 880 GMA E45
RAM: 4x2GB Corsair
GPU: HIS ICEQ Radeon HD 4850
PSU: ACE Power AP550A ATX12V

Problem starts when i upgraded my GPU to XFX Radeon HD 6950
Everything stays the same but i switched GPU and also PSU to Venom RX 500w (because it has 2x6 pin that the new card needs) the ACE Power PSU only has 1x6 pin.

All went well, played a game for a bit... left the room to eat.
when i went back, PC is dead, it smells a bit burnt (not badly.. but that.. hot adaptor smell basically)
Venom PSU was very hot to the touch, GPU normal heat.
I figured that the PSU probably can't handle the new GPU (500 watt too low). took it out,
replaced it with the old Ace Power PSU, replaced the GPU to the old 4850 too.
Won't turn on.

Tried various configuration (no GPU, no Ram, 1 RAM, no CPU)
and basically i can only get the fan to turn on for more than 1 sec if i don't plug in the 4pin power supply for the cpu (whether or not there is any cpu connected to it)
Also tried taking the mobo out of the case and putting it on a piece of woodboard(in case it shortcircuit with the case). same result.

Most likely when the PSU overheats it killed something(s) with it? the mobo maybe?...
any idea what i should do next? or just take it to shop?

Thanks for any feedback.. i'm so sad =( was looking forward to the new card.. hic.
should've bought a new 800w+ psu to go with it.

I would try a know working PSU the ace may not be good.