Suggest best MoBo from this
HITACHI Deskstar 7K1000.D HDS721010DLE630 (0F13180) 1TB 7200 RPM SATA 6.0Gb/s 3.5" Internal Hard Drive -Bare Drive Cache: 32MB - $120
Seagate Barracuda ST31000524AS 1TB 7200 RPM SATA 6.0Gb/s 3.5" Internal Hard Drive -Bare Drive Cache: 32MB - $125
Western Digital Caviar Green WD10EARX 1TB IntelliPower SATA 6.0Gb/s 3.5" Internal Hard Drive -Bare Drive Cache: 64MB - $130
CORSAIR XMS3 8GB (4 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666) Desktop Memory Model CMX8GX3M4A1333C9 Cas Latency: 9 - $47
or G.SKILL Value Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10600) Desktop Memory Model F3-10600CL9D-8GBNT Cas Latency: 9 - $35
or Patriot 8GB (2 x 4GB) 204-Pin DDR3 SO-DIMM DDR3 1333 (PC3 10600) Laptop Memory Model PSD38G1333SK - $34
or Kingston 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1066 (PC3 8500) Desktop Memory Model KVR1066D3N7K2/8G Cas Latency: 7 - $38
CORSAIR Builder Series CX430 V2 (CMPSU-430CXV2) 430W ATX12V v2.3 80 PLUS Certified Active PFC Power Supply - $45
NZXT M59 - 001BK Black Steel ATX Mid Tower Computer Case - $50
Rosewill DESTROYER Black Gaming ATX Mid Tower Computer Case, comes with Three Fans-1x Front Blue LED 120mm Fan, 1x Top 120mm Fan, 1x Rear 120mm Fan, option Fans-2x Side 120mm Fan, 1x Top 120mm Fan, 1x Bottom Fan - $50
PSU (With Power Supply)
Rosewill R103A Black Steel ATX Mid Tower Computer Case with 350W 20+4 pin connector Power Supply - $40
LOGISYS Computer CS305BK Black Steel ATX Mid Tower Enhanced SOHO Computer Case 480W Power Supply - $30
APEVIA X-Dreamer ATXB3KL-BK/420 Black Steel ATX Mid Tower Computer Case 420W Power Supply - $35
AMD A6-3500 APU would be fine you don't need to go Quad-Core since you'll only be having 3-4 programs running at a time (media player, browser and download client)
HDD: Green Drive would be fine for HTPC, however if your not too concerned with saving electricity, since the SeaGate Barracuda 7200rpm is the same price, id probably just go for that. (Also bare in mind the green drive is likely to be slightly quieter).
Memory: 8GB is unnecessary, 2x2GB 1333MHz sticks would be more than enough.
Case: Pretty much anything you like the look of that can house whatever motherboard size your looking at. There's some really nice HTPC cases you can buy.