David Avery used to keep half his 401(k) account in bonds and cash. Last year, the 35-year-old computer programmer moved his entire portfolio into stocks, with one recent purchase being Enanta Pharmaceuticals, a small biotechnology company that posted a loss last quarter.

"I could see it going up maybe 50% at a minimum, just being conservative," says Mr. Avery, who is the co-founder of an investment club in Augusta, Ga. "If the market does well, I could see it double easily."

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