xfx 7950 gx2 is not working

Wrong...a 7950GX2 WILL run on ANY chipset...
on the topic...check the cables...check the Graphic Card on the other system...lots of possibility...maybe the card is damaged...  

You don't say what monitor you are using with this card. It is possible that your card is bad, it is possible it is driving at a refresh rate or resolution your monitor does not support. (Maybe P&P mis-identified it for the card.)
It's kind of hard to diagnose since most ways to do that require you to see the diagnostics on the monitor.

Like the last poster said, check your cables, check the connectors for bent pins, remove and re-seat the card. Since power for the card at least partially comes from a connector exterior to the busplane, it may power the fans without the card actually working.

Also, if you have an analog monitor, and the card has 2 ports, sometimes only one of the ports actually supports analog. Make sure you are on the right port.

Just because the fan is spinning does not mean you have a properly working card.

This was the case with my friends brand new XFX 7900GTX, fan would spin but no screen display. Ended up being a DOA card! we verified this by putting my X1900XT (which he should have gotten btw   ) in his computer only too see that everything worked perfect.

We built our computers at the same time and the look on his face was so funny as I happily played away with my new rig while he stared at his blank screen.  

i tried to re-seat the card for 4 times already but it is still the same. hmm... what do you mean by DOA? i dont understand it..

Dead On Arrival.

Thats what you have my friend.

I went to the store and have my card checked. The card is working perfectly, so the reason is my motherboard. I'll buy a new mobo but I'm not sure what will work, I don't want to end up to another motherboard compatibility issue. I know, I know the site of nVidia have a list of compatible mobo but its full of high-end boards, I don't have enough money for it. So I want to buy wisely, nice mobo but cheap.

I want to buy a mobo that will be compatible for Core 2 Duo, and have a DDR2 slot, and of course will be compatible for my 7950gx2 card. Thats all..

I'm asking for help, please respond.. Thanks

... please help? someone?

tell us more about your PSU, how many amps on 12V (its unlikely that will be a problme, but tell us anyway).

Have you plugged in the power supply to the card?

According to the Nvidia compatibilty list (http://www.nvidia.com/content/geforce_gx2_sbios/us.asp) your mobo does not support that card, so like mpilchfamily said swap it for a 7900GT or replace your mobo, the choice my friend is yours.

700w, ya.. i plugged it properly..

700w, ya.. i plugged it properly..

and how many amps on 12V...

A previous poster mentioned your mobo was not on the compatable board list, so i'd suggest that'd be the first place to look.