Google’s buy of DoubleClick has analysts predicting a flurry of activity in the Internet ad sector.

Shares of aQuantive, the closest competitor to DoubleClick, rose more than 12 percent to $32.01 in Nasdaq trading, one day after Google agreed to acquire DoubleClick for $3.1 billion in cash.

Other Internet ad companies, including 24/7 Real Media and ValueClick, also rose on the news.

J.P. Morgan analyst Imran Khan said Seattle-based aQuantive could fetch as much as $39 a share, based on DoubleClick’s valuation. At least four other analysts upgraded the stock.

Speculation is rife that Microsoft will take a look at aQuantive after losing to Google in the heated bidding.

However, there’s a hitch: aQuantive owns interactive ad agency Avenue A/Razorfish, which would not fit with Microsoft’s existing online business.

DoubleClick and aQuantive dominate the ad-serving market. Both provide software that advertisers, Web publishers and agencies use to deliver advertising and track the success of their campaigns.