WTF! Why should you buy her the laptop?
I can understand you trying to be nice and stuff, but trust me I am talking from experience, keep your hands in your wallet, don't let girls exploit the kindness in you, this isn't a joke $ 500 - 600 is some seriouse money to be just blown away on a GF's gift, you are not even fckn engaged!
And WTF, her brother now is getting picky and now you have to foot the bill! That's unheared off!
SokaMoka has warned you, the rest is up to you.
But If you really want to show you care, here is what you do :
1- Find how much would the laptop she would need cost.
2- Tell her that you would be willing to pay up to a max of maybe $ 150 of it's price as a compensation for her gift(s).
3- Go around looking for hot deals find her the best deal and save her tons of money on the purchase, in a way that the Hot Deal + you assistance
will make it pretty affordable.
In that manner she would get her gift from you, learn how to depend on herself and the spoiled bro that wants a dual core gaming monster while his sister needs a lappy for college can learn a lesson or two.