To answer the easy question first: If I purchase slower ram now I assume it will be compatible but operate at a slower speed? Most likely, yes. Intel's chipsets usually support about 3 memory speeds (top speed and two lower).
What memory will the C2D MBP need? It depends how long Apple waits to release the C2D MBP. If they release it before Santa Rosa (which is expected to be released in the first half of next year), then you'll need DDR2-667 (also known as PC2-5300 or -5400). If they release it with Santa Rosa, it may support DDR2-800 to match the new, faster front side bus. Or Santa Rosa may still top out at DDR2-667; Intel hasn't said anything final about memory speeds.