Hi there,
 
        May I begin with my PC's configuration:
-Dual-core opteron 165 1.8 ghz oc to 2.7 ghz
-2*1 GB DDR400
-2*160 GB SATA II hdd's
-VGA 8600GT  
-internal tv tuner
-ASUS A8N-SLI SE mobo
-2*case cooling fans
--PSU Raidmax 420w TruePower
 
Today I bought an MSI8800GT zilent - (it needs an extra cable to pe powered)- When I powered it for the first time PC froze in just 5 seconds..... I restarted it over 10 times.....I also reseted my BIOS, but with no luck....it stops at the ASUS logo, it doesn't even respond to ctrl+alt+del. After an hour I noticed that the four ATX 4pin-12v connector located on the top/felt side of the mobo is fried. I took me a while to unplug the pins from the connector. I borrowed another PSU to test my mobo (Antec 430 W), I didn't even connected the hard-drives. Same thing.....it was clear, my mobo is fried. Tomorrow it will be replaced.
My raidmax PSU still is still delivering power to all components....is it possible that the only thing that affected was the mobo?....could the PSU not have been affected?......I still need it 'till gather enough money to buy a new one....I promise I will not overclock and i will not use the extra cooling fans till then            

Wo i dont no it's risky it cud brake sumthing alce not good

the 1st thing i do when check old hardware is smell the hardware.  sounds funny but if it smell burnt it might be burnt, but not always.
the psu is made in away to protect the other hardware from things like this.
example: friend was powering his house w/gas generator and plugged into the generator then powered it on, boom all the lights blu and his computer wouldnt start.
he calls me to look at it, funny thing is he and the others he told about this said to thro the computer out.  i look at it and it was only the psu had blown, and i replaced the psu and booted the cpu.  problem solved.
 
event though the psu will smell burnt from the mb, i would give it a try.  also, if your getting power to the rig is not your psu.

PSU doesn't smell of any kind....I know the smell of fried hardware . Like I said, I tried with another PSU and it's the same story. Pc freezes in 5 secs.
Thanks. I sure hope my PSU survived

Looks like your PSU. Next time get a good quality PSU. Get a top tire PSU listed here:
http://www.xtremesystems.org/forum [...] p?t=108088

Replace the PSU any way (even if it is not the problem) esp. since it  is a low quality one.