With Internet valuations surging and bankers circling Silicon Valley, online-games maker Zynga Inc. is preparing to test investors' appetite for an initial public offering that values the young company as high as $20 billion.

That would be double the price tag the San Francisco start-up, which makes "FarmVille" and other simple games played on Facebook's website, fetched just a few months ago when it sold shares to venture capitalists.

But...