  A multi-orbital model for a CoO_2-layer in Na_xCoO_2 is derived. In this
model the kinetic energy for the degenerate t_2g-orbitals is given by indirect
hopping over oxygen, leading naturally to the concept of four inter-penetrating
Kagome lattices. Local Coulomb interaction couples the four lattices and an
effective Hamiltonian for the interaction in the top band can be written in
terms of fermionic operators with four different flavors. Focusing on charge
and spin density instabilities, a big variety of possible metallic states with
spontaneously broken symmetry are found. These states lead to different charge,
orbital, spin and angular momentum ordering patterns. The strong superstructure
formation at x=0.5 is also discussed within this model.
