Pc turns off and on itself with new VGA?!?!

Hello guys, i hope you all can understand me because my english isnt so good.

My problem is this: PC worked fine with old VGA (sapphire ati radeon 4870 HD) and i placed a new one (xfx ati radeon 5750 HD) and the PC goes mad!
Thought the PSU (400W) is the problem and I bought a new one (600W) because the minimum system requirements for the new VGA is 450W!
It confuses me: when i put out the new VGA and place the old one in PC works fine (i have so a another onboard VGA and with it the PC works fine)!
Took the new VGA in a another PC (from my friend) and it works fine. He has very similiar hardware like me.

I have 16 times checked and re-checked that all is on his place, unpluged all cabels and pluged back with no results!!!

My PC confing is:

MB: Gigabyite GA-M61VME-S2 rev. 1.0

CPU: AMD Athlon 64 x2 4800+

RAM: DDR2 2gb mushkin 800 mhz

VGA: onboard NVIDIA GeForce 6100 nForce 400
OLD: SAPPHIRE ATI Radeon 4870 HD
NEW: XFX ATI Radeon 5750 HD

HDD: two SATA drives, one is Seagate baracuda 160 GB and the another one... i thing a
Western Digital.


Please help me, if any one know what the problem might be let me know.

Sorry again for my bad english