Now that investors have had a couple of days to enjoy Sirius XM Radio (NASDAQ:SIRI) perched above the $1 mark, it's time to ponder the stock's next milestone.

$1.50? $2? $3?

We can't get ahead of ourselves. Sirius XM's stock broke through the $1 ceiling with no material news this week. Those same volatile winds can also bring Sirius XM back down.

$0.75? $0.50?

One of the most encouraging signs for the satellite operator's continued trading success came from what many likely perceive as a non-event. Shortly after yesterday's close, Sirius announced that it would be releasing its fourth-quarter results next Thursday.

Companies set quarterly reporting dates all the time. Why is this ho-hum news so tantalizing? Well, Sirius XM still isn't in full compliance with Nasdaq's listing requirements. Shares need to close at or above the $1 mark for 10 consecutive trading days to silence the delisting threat. Sirius XM could have waited until well into March to discuss its fourth-quarter results, but chose to do so Thursday morning -- just six trading days into its dollar-day streak.

A disappointing report, or any kind of bombshell, would derail the stock (if it's still trading at $1 or higher at that point, of course). In other words, the company probably knows it has at least a decent report on its hands, and potentially a great one.

This doesn't necessarily mean that the sky is the limit. Even at $1, Sirius XM trades at a steep price-to-sales multiple when pitted against satellite television giants DirecTV (NYSE:DTV), and just a bit below that of DISH Network (NASDAQ:DISH). It's a multiple that only gets wider when you consider Liberty Capital's (NASDAQ:LCAPA) 40% preferred share stake, or tack on the company's long-term debt to calculate its enterprise value.

Sirius XM is making some serious inroads in free cash flow and should be profitable within a year. However, DirecTV and DISH are already consistently profitable.

This doesn't mean that Sirius XM is overvalued today. It simply needs to prove that it can either expand its subscriber base or increase what the average subscriber is paying.

How big is the market?
Sirius XM is the only name in premium radio, and it benefits from having deals with all of the major automakers. When Ford (NYSE:F) or GM is pushing factory-installed Sirius or XM receivers -- because there are financial incentives to do exactly that -- it can only help the subscriber acquisition process.

The key here is assessing the size of Sirius XM's potential market. There are far more cars out there than the nearly 18.8 million subscribers that the company boasted by year's end. Every car represents a potential paying customer, but it's not feasible to bank on too high a penetration rate.

Cable and satellite television sells because most people spend several hours -- if not more than half the day -- at home. How long have you been driving today? At $15 a month, satellite radio is considerably cheaper than cable TV, but terrestrial radio and the growing proliferation of web-tethered gadgetry offer a broader choice of free audio entertainment for skinflints. Between Pandora, CBS' (NYSE:CBS), and Apple (NASDAQ:AAPL) iPhones and iPods, the dashboard is getting crowded in a connected future.

But who says that Sirius XM will be limited to cars, or even North America? Not only does the company sell portable receivers and home-docking stations, but the availability of online streaming pushes Sirius XM's signals through a variety of devices -- and possibly down the line -- abroad, beyond the range of Sirius XM's army of satellites.

It's a cutthroat world in cyberspace -- and Sirius XM may have to slash prices to truly penetrate the global market -- but it opens up the potential for serious upside.

Milking lessons 101
The other catalyst that could light a fire under Sirius XM's stock is the ability to generate more revenue from the average subscriber.

There are currently upsell opportunities in navigation and limited backseat video, but they don't have to end there. Creating more premium channels is one possibility for expanding revenue, though the company's best bet for per capita revenue expansion may rest on broader online access.

Connectivity will be a major theme in the coming years. This may pose a competitive threat to Sirius XM by leveling the playing field, but it's also an opportunity. Web-connected programming will open the door to e-commerce and on-demand content.

In other words, Sirius XM doesn't necessarily need to hike rates, even when it's legally entitled to do so next year, to make more money from the typical subscriber.

Up vs. down
So which way does the stock go from here? Higher? Lower? Let's crack open a poll to see what you think.

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Longtime Fool contributor Rick Munarriz is a subscriber to both Sirius and XM. He does not own shares in any of the companies in this story. He is also a member of the Rule Breakers analytical team, seeking out the next great growth stock early in its defiance. The Fool has a disclosure policy.