Need a budget motherboard for gaming

Hi!
A friend asked me to build him a new rig, the cheaper the better. Strictly for gaming on a 19", no OC. His current PC is like 7 years old,

That's what I have this far :

Intel E6300 LGA775 2.8GHZ 1066FSB 2MB
OCZ Gold XTC PC2-6400 4GB 2X2GB DDR2-800 CL5-5-5-18 DDR2 240PIN
Western Digital WD5000AADS Caviar Green 500GB SATA 32MB 3.5IN
Antec Three Hundred Mini Tower Gaming Case 300 ATX 3X5.25 6X3.5INT
Corsair CMPSU-400CX 400W
Sapphire Radeon HD 4670 750MHZ 512MB 128BIT GDDR3 PCI-E HDMI DVI-I VGA

I really have no idea about the motherboard. I'm wondering if mATX (I don't know much about that) would work with those pieces ?

I use the Antec 300 myself with a P5Q Pro and a HD4870, it's tight but it fits, so I'm not worried about space.

Anyone could help me choose, that'd help me alot!

Thanks!