The sell-off might be scaring investors, but this is a good time to get greedy.Nvidia (NVDA 3.54%) stock has led the AI stock boom since the launch of ChatGPT, but there are now legitimate questions about whether that rally still has steam.After the company reported earnings on Aug. 28, the stock fell even though it beat analyst expectations, a sign perhaps that Nvidia's valuation had outrun the growth in its business. Last week, the stock tumbled again, first on a report that Nvidia later denied that the c ...