Dual-core for web surfing? Overkill. A single core would do, but it is only a $31 difference.
LG? Like this one:
NCIX.com - Buy LG GSA-H55N Beige DVD+RW 20X8X16 DVD-RW 20X6X16 INT IDE 2MB OEM DVD Writer W/ SW - GSA-H55N BEIGE In Canada.
Most mATX cases use ATX PSUs. At least the mini-towers do.
2 gig will be plenty of RAM.
Building a mATX is just like a ATX, just not as much room.