>That's a very big if...
Oh, this is indeed a big if. A large, looming aspect of the problem is we don't anything like an exact characterization of "general intelligence" so what we're aiming for is very uncertain. But that uncertainty cuts multiple ways. Perhaps it would take 100K human-years to construct "it" and perhaps just a few key insights could construct "it".
> Also, I'd argue that most progress happens not because of some brilliant people, but because of many people working together...
The nature of a problem generally determines the sort of human-organization one needs to solve a problem. Large engineering problems are often solved by large teams, challenging math problems are generally solved by individuals, working with published results of other individuals. Given we're not certain of the nature of this problem, it's hard to be absolute here. Still, one could be after a few insights. If it's a huge engineering problem, you may have the problem "building an AGI is AGI-complete".
> Then if your AGI only reaches the level of intelligence of humans and maybe a bit more (what does more even mean in terms of human intelligence? more emphatic? faster calculation ability?
I've heard these "we'll get to human-level but it won't be that impressive" kinds of arguments and I find them underwhelming.
"What use would more memory be to an AGI that's 'just' at human level?"
How's this? Studying a hard problem? Fork your brain 100 times, with small variations and different viewpoints, to look at different possibilities, then combine the best solutions. Seems powerful to me. But that's just the most simplistic approach and it seems like an AGI with extra-memory could jump between the unity of an individual and the multiple views of work groups and such is multiple creative ways. The plus humans have a few quantifiable limits - human attention has been very roughly defined as being limited to "seven plus or minus two chunks". Something human-like but able to consider a few more chunks could possibly accomplish incredible things.