Thanks for your help !!!
As you all suggested, went to my vendor gave him a nice SMILE

and said can i like... Exchange this RAM stick?
He did asked why? I told him "cause bla bla bla..."
The Vendor complied (at the same time, was quite "OHH!!! really.... hmmmm")
Anyways... I have to say one thing!!! since i got that AGP card "Geforce 6800 GT" my system was freezing (both old one and this new one)
But after i have put 2 Ram of same type (hynix 400 DDR) = Dual thingie
Nothing has happened yet... (fingers crossed)
Do you think, it could have been my AGP card all the way ? Slowing and slugging my PC ?
Thanks