inb4 Nvidia vs ATI and AMD vs Intel flame war

go for 8GB for gaming, more is only needed if you do video editing or have many applications open at the same time; since I'm a fanboy I'd recomend Nvidia over ATI and AMD over Intel ;-)

Now, you can use this website to make an informed decision, click on the component you want to get and it will tell you the average price and how much value you get for it compared to 10 others depending on what you are looking for like performance over price or viceversa:

http://www.cpubenchmark.net/
http://www.videocardbenchmark.net/