Minutes after the Dept. of Justice approved Sirius Satellite Radio Inc.’s (SIRI) merger with XM Satellite Radio Holdings Inc. (XMSR), each company’s shares soared by double-digits.  Yet few analysts know with any certainty whether that sharp jump will continue.

Whether now is a time to jump in with both feet… or whether the surge will cool on the heels of valuations soon to be posted by analysts, who no doubt are bleary-eyed and caffeine-drunk from crunching the numbers through the night.

So the question is, should you buy?

The answer is not an easy one. It depends on three factors.

To begin with, the merger – which was first proposed in November 2007 – still has to pass through the Federal Communications Commission, which could impose conditions on the merger that could vastly change and agreement and/or favor the groups the lobbied heavily against it.

The FCC is expected to rule within the next few weeks. April Horace, an analyst with Janco Partners who has been following Sirius, said the FCC shouldn’t take too long to ponder.

“It’s not like this hasn’t been sitting on the FCC’s plate. It’s been there for a year,” Horace said.

In its ruling, the Dept. of Justice said that a Sirius/XM merger would not violate anti-trust laws because the satellite radio providers face competition from AM and FM broadcasters, television, mobile phones and online music stores and radio stations.

Secondly, the merger still faces tough opposition from one of the most lobbies in the world. Many of Sirius and XM’s competitors are part of the National Association of Broadcasters, one of the merger’s most prominent opponents. And looking at its executive committee – which includes senior managers from billion-dollar media titans such as Hearst-Argyle Television Inc. (HTV), Gannett Co. Inc. (GCI) and Belo Corp. (BLC) – it’s no wonder why the DOJ investigation took so long.

“We are astonished that the Justice Department would propose granting a monopoly to two companies that systematically broke FCC rules for more than a decade. To hinge approval of this monopoly on XM and Sirius’s refusal to deliver on a promise of interoperable radios is nothing short of breathtaking,” The National Association of Broadcasters said in a statement.

Third, will this merger give the combined companies the leverage they need to reduce operating costs and increase profit?

The merger would likely combine XM and Sirius’s content together for a lower price. Subscribers of both now pay $12.95 a month, but may pay as little as $6.99 with the proposed tiered pricing, Bloomberg reported.

Sirius has 7.67 million subscribers who have access to premium content that includes talent such as Howard Stern and NASCAR broadcasts. While XM’s 8.57 million listeners pay a premium for content from Oprah Winfrey, Major League Baseball and in-studio artist performances.

To fund such content and compete with each other, both Sirius and XM must spend millions of dollars a year.

Together, they can combine their best programming and management and form a music and news medium better equipped to fight against billion-dollar competitors such as Clear Channel Communications, Inc. (CCU) and Apple Inc. (APPL).

XM shares closed at $13.79 yesterday, going up $1.85 for a 15.49% gain. Sirius shares closed at $3.15, up $0.25 for an 8.62% gain.

Shareholders already approved of the merger. More than 96% of those who voted approved the transaction in separate meetings in November, Bloomberg reported.

But again, a large part of the final deal depends on what stipulations, if any, the FCC lays forth.