Taking a look at last generation console install base wich is about 200 million, I don't think the current market is going to expand significatively, another question is the portable market NDS + PSP = 155 million vs GBA = 80 million.
So extrapolating last gen install base my estimation would be:
PS2 (145 million) ---------- Wii (115 million)
DC + Gamecube + XBox (55 million) ---------- XBox 360 (55 million + PS3 45 million) = (100 million)
Last gen final sales (200 million) ---------- Current gen final sales (215 million)