You really can't go wrong with neither an AMD 7870 or an Nvidia 660 Ti cards--both around $200 and can ran at full 1080P with high settings with every game out there. Both cards are around the $200-225 range.
Just about everyone recommends the I5 (quadcore) series for CPU ($150-200 depending on model).
I would go with 8GB 1600 DDR ram though, you can get them cheap $50-60.
For PSU get a brand name (generic ones seem to die or not carry the correct voltage). You can get a good Brand name PSU for about $50 to $60 for 600 watt models.
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A big part of me says to wait for the first UE4 game to release prior to building an gaming PC. Usually when a new engine is released a flood of new graphics cards come to the market and you usually can score a better deal that way and also future proof your system since engines tend to last a few years at least.
Also why the sudden need for a gaming PC? The PS4 is an x86 system which means just about everything that's going be released on the PC will likely coming to the PS4. Might as well get an PS4 and save yourself $500 for more games

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