I'm looking at building an HTPC for the living room, having never looked at HTPC's I'm a little lost, so if anyone can offer me some guidance on the following points I'd appreciate it 
The main uses are for streaming 720p videos and programs from my main PC upstairs to watch on the TV so it must have wireless N networking and a HDMI output, aside from that I'm open to suggestions on what else is required.
1. The spec, I need a case and all the components. I have two PSU's sitting unused from older PC builds, if possible I'd like to reuse one of those but I will need to check connections.
2. The software, what's the best software for a HTPC?
Cheers
Seppy

The main uses are for streaming 720p videos and programs from my main PC upstairs to watch on the TV so it must have wireless N networking and a HDMI output, aside from that I'm open to suggestions on what else is required.
1. The spec, I need a case and all the components. I have two PSU's sitting unused from older PC builds, if possible I'd like to reuse one of those but I will need to check connections.
2. The software, what's the best software for a HTPC?
Cheers
Seppy