Phenom II X4 955 - compatible with this ddr2 800 ?

That will depend on the motherboard. You must have a AM2+ mb. Yes if the board will support the ram and the cpu it's a go. That cpu will support up to DDR3 1666 on a AM3 mb, possibly more. That is mb dependent.

Thanks a lot for answering so fast  

I will use this motherboard

Gigabyte MA790GP-UD4H AMD790GX ATX AM2+/AM3 Sideport 2PCI-E Sound GBLAN HDMI CrossFireX Motherboard




So i guess it will be ok right : ) ?

yes the ram would be fine.