Motherboard -
Gigabyte GA-M61P-S3 w/onboard GeForce 6100 or BIOSTAR TForce 6100 w/onboard(Since you wanted Micro ATX)
CPU -
AMD Athlon 64 X2 3600+ Brisbane RAM -
1GB G. Skill DDR2-800 Power supply -
FSP Group (Fortron Source) 400W Hard drive -
WD 160GB hard drive Optical drive -
Sony NEC Optiarc DVD Burner
These parts come to a total of $316.94, shipping price not inluded. Just choose a case to your liking. Maybe something like this for the case, since you wanted it as small as possible:
Newegg.com - APEVIA X-QPACK-NW-BK/420 Black Aluminum 1.0 w/ ABS plastic front panel MicroATX Desktop Computer Case 420W Power Supply - Retail
With that case, your total would come to $386.93