i just downloaded the game and played for a small time... i read a blurb somewhere that you can choose to not have help from an old lady and that doing this will make the game more tense and paranoid. was it referring to the part in the convo where you can say something like "fine, i will do anything you want" or "i don't take orders from anybody, i will do it my way"? and what are the major gameplay differences in these choices?

also, a smaller question, how do you save over a save file? it will let me use the arrow keys to select a previous save to save over, and when i just use the same name instead of saving over it just left me with to save files with the exact same name.

That choice should matter (you need to refuse her to get the golden ending) but it seems the trigger doesn't work and it's only a role-playing decission.
You need to delete old saves manually - they can't be overwritten :( The dev tried to fix this, but messing with save system messes up the game.

The first coice early in the game are only cosmetic, as the old lady won't show up much. There's a very small scene after the first parasite and you'll meet her again near the end of the game.
It's the last encounter where you have to refuse her help, if you want to get another flag for the best/golden ending.

Actually, I couldn't get the golden ending during my first playthrough (replayed chapter 7 several times), so I checked online for what decisions I should have made...
to my knowledge the only thing that I hadn't done correctly was to refuse the queen of maggots in chapter 1, so I had to start again from the beginning.
Idk, maybe Novotnus is right and the trigger doesn't work and I just remembered wrongly, but still - if you want to make sure to get the best ending I think you should rather refuse her help, it doesn't make any difference gameplay-wise anyway..

So to conclude - this choice only impacts the ability to get the "golden ending" and nothing else, right?

Yep, it definitely doesn't matter for how the game unfolds, everything happens exactly the same way, except of course for the dialogue that immediately follows (she's kinda pissed if I remember correctly ;) ).