More Necron fluff
I'm reading Hammer & Anvil at the moment and came across some info that might help clear things up:
It's stated in the book that Necron ships are running around the galaxy "traveling at near-light speeds." Ok, so they don't have any FTL. Lovely. Also, Dolmen gates are a rare thing and not easily repaired(though they can be, given time. In the book it's been many years since the Sanctuary 101 incident and the thing still isn't fully fixed.) Nothing at all on if more can be created, but since the Necrons go to such lengths to find and fix lost ones I'd assume not. They appear to be the Necron equivalent of STCs in terms of "valuable relics of a lost age" type stuff.
So Necrons essentially move slower than the Tyranids in normal space and have a handful of Dolmen gates that destroy themselves over time just by their function. This essentially means that any Necron fleet that wants to attack somewhere is going to have to do so in such massive numbers so as not to be destroyed when they finally reach something like a heavily-defended world.
Imotekh now knows how big a Black Templar crusade fleet is and would take steps to not lose a second time, making them a lot like Tyranid splinter fleets(though IMO it seems really unlikely that he should have even lost the first time to some Imperium ships.) Also, I still can't seem to figure out if Necrons can actually make more stuff. They can re-purpose already constructed bodies, machines, stuff like that, but if they could make more armies from scratch I haven't seen any evidence of that.
Oh, and on the subject of what the Necrontyr used to look like, the cryptek Ossuar in the book states that human biology is a recursive evolution of their old organic forms and he was curious to know exactly how that happened. Sounds like they looked almost exactly like humans.
EA: The pattern has repeated itself more times than you can fathom. Game companies rise, evolve, advance, and at the apex of their glory, they are extinguished. Bioware was not the first. By utilizing our funding, game companies develop along the paths we desire. They exist because we allow it, and will end because we demand it.