Back when I was rocking DDR1, I would frequently have multiple manufacturers represented in a system. At one point, I was mixing manufacturers (Crucial, Kingston, and some off-brand) speeds (PC2100 and PC3200) and sizes (512MB, 256MB, and 128MB). Ran like a champ.
DDR2 and DDR3 shouldn't care either, though how closely your mobo and RAM follow (and require the other follows) electrical specs will have an impact in reality.
JigPu