First you should take a look at these two links below. Second its going to be very hard to squeeze an i5 into a 500 dollar build and marests 560 build is probably the most balanced system in that price range. You could replace the 6870 with a 7850 for fairly close to the same price. But if you go i5 you will be unbalancing your system with a budget of 500.
http://www.mmo-champion.com/threads/...-build-Read-me!
http://www.mmo-champion.com/threads/...-Sample-Builds
Budget Gaming 560
MoBo: ASRock H77 Pro4/MVP – $69.99
CPU: Intel i3 3220 – $129.99
RAM: G.Skill 1600MHz 2x4GB – $33.99
GPU: Sapphire Radeon 6870 – $164.99
HDD: WD Caviar Blue 500GB – $69.99
PSU: Antec VP-450W – $35.99 Review
Case: BitFenix Merc Alpha – $49.99
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Estimated Total Price – $565