When I rebuilt my PC, I got the i5 2500k and the nVidia 460 SEv2. At the time they were rated best bang for buck, although not the most powerful. A year later they're still running great, and I haven't even overclocked the processor yet. If you decide to do the separate CPU/GPU here's a few suggestions:
1. Make sure you have enough fans. A standard PC case likely won't have enough and you may run hot. I'd recommend adding 1 extra fan to the interior of the machine to circulate more air.
2. Make sure you have enough power. You'll probably need at least 700 Watts, and make sure you add up what all your components need or else your computer won't start.
3. Although it's not as fun of an OS to use, apparently Windows 8 is best in terms of gaming performance versus 7.
I have an A8 in my laptop. It has run all the games I've thrown at it but I don't play the latest and greatest.
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