Before making any suggestions please look at my current build below...
AMD FX-6100 3.3 GHz, ASUS M5A97 LE R2.0, Kingston HyperX 8GB @ 1600 Mhz, Sapphire Radeon HD 7770 1GB Ghz edition, Fractal Design Arc Midi R2, Corsair CX 500m 80 Plus Bronze

The fields I'm looking to upgrade in are the GPU, Mobo, and MAYBE the Case(?)(Thinking about a smaller case with good cable management capability)

Something I'm not certain about is whether or not to upgrade the processor... as far as gaming (all I do on this pc) should I just stick with that one or maybe get an 8 core (i'm staying with AMD CPU's)... it's a tough decision

Ultimately, I'd like to spend less than $400 on an upgrade that will last me at least until next winter and maybe to the summer of 2015.

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PS. any advice on cable routing in this case? i'm not talented at it...