You have dual channel (two sticks) and quad channel (four sticks) all which must be the same size, and preferably brand and type. I have however heard that using all four DIMM slots will actually downclock you RAM speed.
For example, on some socket 939 boards, I've read that using all four DIMM slots downclocks the speed the 333MHz for some reason.
And you can add memory to any DIMM slot, you just have to do it in the correct order by the colour codes.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w