Help me with chosing parts for my budget gaming build

I am doing a budget build cause i am only 12 and my mom doesn't have lots of money to spend so i have save some money that i earned.I only got about 400ish i know that isn't a lot but i built 3 gaming rigs around that amount of money before and they have be very good.I was on newegg looking around and i saw the phenom ii 2 550 black edition,i read the feedback and most of the people unlocked all 4 cores on it,i want a quad core so should i get that cause i would save some money,the case i want is the Antec Sonata III cause it looks good and got good feedback and it comes with a good powersupply.Should i get that case or buy a other case and a power supply.I also got one 4850 and i might buy a nother one for crossfire.I only paid 60 dollars for it so i got a good deal for it.But basically would you all who reads this help me with choosing parts that will work excellent with the 4850.

Thanks