I doubt Heller will be as big as Roe. Of course, much of that depends on what you view as more important subject matter.
It really depends on how the Court approaches the case. During the latter days of the Rehnquist era, the Court often avoided issuing broad rulings, instead using a more limited rationale to affirm/overrule.
Ideally, the Court comes out and definitively rules that the individual right to own and bear arms is a fundamental right subject to strict scrutiny. If so, I think the DC law will most certainly have trouble on the last 2 prongs.
More likely, I think, is that the Court will overrule the law with similar reasoning, but without establishing individual, unrestricted gun ownership as a fundamental right. I'm not sure what level of scrutiny is generally thought to apply to gun ownership, but I think the Court will find that the law fails based on some lesser standard because it is not reasonably/substantially related to decreasing violent crime.
How do people think this will come out? My off-the-cuff, haven't researched any gun-related positions reaction is that it'll go 6-3, with a more narrow opinion. Breyer or Souter breaking and coming to the majority. Of course, I haven't had opportunity to do any research or even read the amicus briefs, so I'm quite possibly way off.