First off, that laptop seems adaquate to learn on, and that extra memory will definately help speed everything up. The integrated graphics isn’t all that good, but will be adaquate for a learner, I’m sure.
To the second question.
How old is your brother?
How much time is he willing to spend learning computer graphics?
How much does he know about computer workings and any kind of computer graphics, including 2d work like photoshop.
Very few can ever make graphics as good as commercial games, and commercial games have huge teams with each person focusing on a certain aspect, so the answer to that question may be never. I would say after 6 months, if he is very diligent about learning Blender, and works all the way through the Blender documentation, he should be able to make good models, with good textures, but this is so arbitrary… anyway, take it for what it’s worth.
How good do the graphics need to be? That’s all up to you. I can’t really say "Your models need “this number” of polygons to be good. It’s all subjective. How good do they need to be? Good. Show them to us, and I’m sure there will be people to help you figure out what good is.
modeling is one thing, you can be a talent and learn it within weeks but that is very rare. i would rather see it realistic, when you aim for commercial quality game models it will require not only modeling skills to be learned but also he needs to learn what is good modeling. and that will take a lot of time and practise. but when he focuses only on one genre he will be faster progessing. drawing and analysing other models will increase that.
still a year is not a lot, and most students here need at least two years of academic teaching to get to a medium good point.