Especially if you're behind the curve on your retirement savings, your 40s is an incredibly critical decade for your investment plan. You're still young enough to invest aggressively and let compounding do much of the heavy lifting to get you to a comfortable retirement. At the same time, though, you're not so young that your portfolio can easily recover from a major stumble.As a result, you want companies with a good combination of potential growth ahead of them and reasonable valuations when you buy them today.