Athlon II X4 640 Propus vs Phenom II X2 555 Black Edition

Hi this is my first thread on hear and i have question I've been pondering for the last 2 weeks.

I'm building a new pc from scratch minus the disk drive and hard drive that im binging over form my current computer. here's my current build plan.

COOLER MASTER HAF 922
ASRock 870 EXTREME3
HIS H577FK1GD Radeon HD 5770 (Juniper XT) 1GB
RAIDMAX HYBRID 2 RX-730SS 730W
G.SKILL Ripjaws Series 4GB (2 x 2GB) DDR 3

so as you can see i have yet to choose a cpu. Im looking for one under $100 because this is a budget build. I like the Athlon II x4 because its a quad with a lot of power at $99 and i like the Phenom II x2 because its also powerful AND it can potentially become a quad. My mobo can unlock cores so i know that i have a chance at it.

This pc will be a mostly for gaming. I do plan to try over clocking the cores. i will surf the net on it but thats not taxing. So any ways which should i go with? the quad core or take a chance and possibly have a more powerful quad core.

Any advice is much appreciated.