Even for the most cold-blooded investor in modern financial markets, round numbers retain a primal fascination. Trading floors are still apt to erupt in cheers when equity indices top any given thousand mark. So what to make of three big round numbers already out there or in the offing right now? Oil is nudging $100 a barrel. Goldbugs enthuse that $1,000 an ounce for the yellow metal is not too far off. PetroChina has just become the first company ever to hit a market capitalisation of $1,000bn.