I'm calling this an HTPC since it's only connected to the TV but might not fit the standard compared to some of the builds I see here. I currently have an Intel Core2 Quad core 9550 with a fanless ATI 4670 graphic card. This is a midtower with a Gigabyte GA-EP45-DS3R. Up until a couple weeks ago it was a Gigabyte EP45-UD3P with a Intel Q6600 but began having issues with either the CPU or motherboard (not sure which yet). I went from an Antec P180 case to a Coolermaster CM690. These were all hand-me-downs from other projects and decided it was time to get something a bit smaller.
I decided on a Coolermaster Elite 120 RC-120A-kkn1 case as a replacement. The reason being that I wanted an external 5.25 drive bay for an existing DVD drive and wanted something that could take a standard ATX power supply and a couple of 3.5 inch HD's. The price was only $40 shipped from Newegg. The dimensions fit my HDTV stand and it had higher reviews than a lot of other cases there. It's big for an ITX case but small in comparison to my existing ATX case.
For the motherboard, I chose the Intel BOXDH77DF since it was in Assasin's guide as one of the ITX boards that had an Intel chipset for the NIC. I've been playing a lot of videos and stuff from another Windows server so figured that the network connectivity was more important than some of the other considerations. I almost went with the ASRock FM2A85X-ITX board with an AMD trinity setup but in the end decided against it mainly because of the problems ASRock had with the A75 boards and this one was too new to have any type of record. Newegg didn't have any other AMD ITX boards.
For the CPU, I chose the Intel I3-3225 mainly because it was the lowest cost Intel chip to have the HD4000 graphics built in and seemed to be at a good price point for cost versus performance.
For memory, I chose the G.skill Sniper Low Voltage DDR3 1600 (2x4GB) chips. To be honest, I didn't put a lot of thought into this -- they were cheap and seem to have good reviews but may not end up being a good choice.
Total cost so far was $372 including shipping so didn't put a lot of money into this yet.
I'll move my existing DVD drive, 640GB WD hard drive, Corsair HX620 modular power supply, and X-gene 01027 wireless keyboard to the new system. I know the power supply is way overkill for this box but I already have it and I don't think it'll be any bigger in physical size than any other ATX power supply. The OS will be Windows 7 since I also already own that.
The primary use for the HTPC now is simply watching DVD's and HD TV recordings with VLC but may develop into something else over time.
This is my first ITX build -- everything else I've ever done has either been full-tower or mid-tower. So, where have I screwed up?