FM2 Socket might be the way to go... here are some parts
1) CPU/APU - AMD A10 5800k = $135
2) ASUS F2A55-M-LK-Plus Motherboard = $69
3) Corsair CMX4GX3M2A1600C9
4) Antec NCK3480 Micro ATX Case with 380W Power Supply = $109
5) [ Optional any cheap dvd optical drive ] = anywhere from $15 - $29
Total = $380
Should be under 400 hopefully. All prices are based on Australian parts
so should be cheaper in the US.
You can add a 6000 series AMD Graphics Card to it later down the road
for crossfire config and get better frame rates if you want to play more
demanding games; still nothing too demanding.