Go with a radeon hd4850 or hd4870 graphics if you want the most bang for the buck.
As for cpu a 3GHz core2 duo would suffice, I don't think a quad core benefits games that much yet.
I'm pretty sure wireless-N refers to the new draft N (802.11N) standard which allows wireless speeds up to 300mbit, wireless-G (802.11G) will only give you 54mbit.
So it depends on how fast your internet connection is, if you're on 100mbit you wil…