I sincerely doubt that the choices we made in ME1/ME2 will have an effect on whether we can win or lose ME3. The game should be just as playable for the new guys as for the veterans, but it should also provide rewards and consequences for those of us that spent 30 or more hours really getting into the series. Sure, most of our imported glory may be in the background, but that's not necessarily a bad thing, and that doesn't mean you have to give up the possibility of a lose/lose situation in the end, either. Note that, while your actions in ME1 had little impact on the overall outcome of ME2, they did seriously change the environment in which you played. Paragons had more allies, but renegades had caused a much greater shift in power. And, regardless of what you did in ME1, ME2 still gave you the option of either epic victory or complete failure (okay, maybe not COMPLETE failure, since the Collectors were still stopped, but you get the idea), and added a lot of interesting choices to the mix (to genophage or not to genophage, for example).
So here's what I think: How you played ME1 and 2 will determine who your allies are and the overall political environment of the galaxy. Paragons will have created greater unity, and thus will have more races and factions at their sides during the final battle, but they probably nuked the Collector base and gave TIM the finger, so they won't have that much of a tech advantage. Renegades will have far less friends, but humanity will have reached its peak in power, so while you have the smallest final attack force, you get the biggest guns. Equal chance of victory for both, but with a completely different flavor. The "default" Shepard will probably get a mix, where relations with other races are tense, but a few are willing to cooperate, and the Alliance is certainly a major player but is not quite as strong as, say, the turian military. The new kids are missing out on a few goodies, certainly (no awesome weapons based on Collector tech, no alliance with the geth), but they aren't being completely cheated out of a fair fight.
The battle itself, however, should completely rely on what decisions you make in ME3. I can see results similar to the suicide mission: either it is a) a triumphant victory over the Reapers,

a long and hard battle in which casualties are massive and Shepard him/herself dies, but there are enough survivors left to pick up the pieces and start over, or c) you are completely unprepared for the final confrontation, get completely wiped out, and the Reapers make tasty milkshakes out of you and mindless slaves out of all your friends. Hopefully, the decisions that lead up to this will be less about moral alignment and more about strategy and effort.
Of course, if you get ending A, I expect a nice epilogue in which either all the races are sitting in a circle singing Kumbaya (mostly paragon) or humanity is kicking the sh!t out of those arrogant alien dirtbags and ruling the galaxy (mostly renegade). Or tensions remain (neutral), and no one is sure where everyone will end up, but there is no doubt that things will be different from now on.
Note that the paragon/renegade outcomes are less about good and evil, but more about doing the RIGHT thing or doing the SMART thing. The typical paragon, while level-headed and diplomatic, still makes decisions based on their own morals; the average renegade, on the other hand, is a selfish, violent, racist buttmunch, but often chooses the most practical solution. Interesting how that works out, really, with the emotional ones using their brains and the logical ones thinking with their hearts... You'd think it'd be the other way around.