Entertain the newbie!

[cidgregor]

Okay, so I am more of a fan of the anime than the game-universe, and as such I haven't played any of the games since the original red/blue/yellow. But today I finally broke down, if only for the nostalgia factor, and found myself buying Leaf Green, and am now forced to actually give a crap about all the newer-generation additions and adaptations. So I'm basically going to sound off a couple of random wonderings that I'm having at first impressions, and ask that y'all forgive the newbishness of them:

Firstly, I'm curious exactly how stat-increase rates and stuff change across evolutions. For sake of example, let's talk about one of each of the two primary evolving methods: Squirtle (evolving by level) and Pikachu (evolving by stone). I'd get into great detail about exact stats and stuff, but I'm short on time, so in brief: what's the 'ideal' time to evolve a pokemon in these two situations? Will stats boost faster at higher evolutions, thus making the ideal evolving times simply "ASAP," or should I wait for some particular key moment to maximize them?

And secondly...I'm completely new to the concept of pokemon's 'natures,' and I'm curious: are the differences they create all that significant? Like, enough to frequently be a deal-maker/breaker in anything from battles to trades?

Thanks in advance for catering to my ignorance.