I'm building a budget computer that can be used for gaming.
Currently have these parts (just bought them yesterday on black friday):
CPU: i5-2500K ($150)
PSU: CORSAIR CX 500 Builder Series V2 ($40 after rebate)
MOBO: ASRock Z68 Pro3-M 1155 MATX ($50 for a bundle deal) But I will try to switch this for P8Z68-V LX 1155 ATX ($60 for a bundle deal) since I don't want a mATX mobo.
RAM: CORSAIR 8GB DDR3 1333MHz Dimm Kit ($25)
Pretty average for a $500 computer and you shouldn't have any problems with it.
I don't see any good reason to get rid of the micro atx motherboard unless you want to x2 or more on video cards which should be unnecessary with the already good 6870 and is limited by the Corsair 500w power supply anyway.
If you insist on switching the motherboard, you should also switch the PSU since the only reason to switch is for more video cards and it would just be wasteful to have to buy 2 PSUs.
You only need 2x4GB RAM sticks if you intend to use Windows 7, if you will have XP on this then just get something with 2x 2GB RAM sticks instead, because XP won't recognize anything more than that anyway.