Well first, I'll suggest you use newegg. They have better prices then a pre built pc.
You could go for the X99 Motherboard for DDR4 RAM And the new processors. This may cut into your FLU Budget but it is well worth the new LGA Processors.
Go for NVIDIA Graphics cards. You Could go all out with the. Titan z or a base titan. But for gta and minecraft,go for a GTX 980. Power supply should be at least 1KW and RAM at least 8. 16 is recommended.