You are looking for a "mini-ITX" motherboard.
Your main decision will be the CASE. I strongly recommend you buy one that supports a normal ATX power supply.
Here's the best mini-ITX board:
http://www.ncix.com/products/?sku=71116&vpn=P8Z77-I%20Deluxe&manufacture=ASUS&promoid=1360
Here's a good case:
http://www.ncix.com/products/?sku=71144&vpn=BFC-PRO-300-WWXKW-RP&manufacture=BitFenix
Here's a VIDEO on a gaming build:
http://www.youtube.com/watch?v=5iRzZmudc4M
*Notes:
- get an after-market CPU heatsink/fan and make sure it both FITS and uses a PWM fan
- make sure the motherboard has PWM fan control for the CPU
- The Corsair CX430 is known to have noisy fan issues (I bought one). If you're building a gaming rig with a good graphics card you likely want a 650W PSU anyway.
- make sure any RAM you choose isn't too tall for your CPU heatsink
- 8GB of RAM is plenty for a gamer (video or intensive photo editing may benefit from more) and 1600MHz, low-latency RAM is recommended
- whichever graphics card you use, make sure it has a good cooling solution
- *triple-check SIZE and CLEARANCE
- Windows 8 64-bit OEM, plus Start8 from Stardock is recommended
- Intel i5-3570K is the best gaming CPU
- 1155/Z77 motherboard is recommended