Seasonic PSUs are the only power supplies that have consistently performed well for me in all my builds. Whenever I try a different brand, I always end up going back to Seasonic.
If you need strictly headset, the Kingston HyperX Clouds are fantastic. I have the first model and love it. They just came out with the new version that has the virtual surround.
Yamaha, Pioneer, and Denon are good places to start. Figure out a budget and start from there.
About Onkyo, personally I've never had issues but I've talked to many people who've had massive issues with there Onkyo receivers.
That onkyo is a good receiver and will go a long way.
As for speakers, if you are looking for floorstanding speakers that aren't expensive the Pioneer SP-FS52-LR are very good.
They sell them at brick and mortar stores so you can demo them if needed.
One thing you may want to check is the...
If you are just using it for playback, an AM3 build would be enough especially on a budget.
Either i3 or AM3 builds you will not need the low profile card cause the built in GPUs will handle decoding the video just fine.
Only reason I'd consider an i3 build is for decoding h264 10bit files...
With those requirements its best to get a receiver and 5.1 speakers.
A midrange Pioneer or Yamaha receiver and maybe the Monoprice 5.1 system would suffice.
1. I would go with power efficient drives. With HTPCs you want a quiet drive.
2. I don't have any experience with tuners yet. I'm in the same boat as you. From what I've read, it comes down to which OS you are going to use. Since I'm using OpenELEC I'm limited on tuners I can use.
3. 4gigs...
Had some extra parts laying around so got a slim case and ssd.
i7 750, 4gigs RAM, GT640, and 64gig SSD running OpenELEC.
I'm using my iPAD as the remote.
Just got mine from Amazon (must be new stock).
Colors are very nice coming from a U2410.
My backlight bleed is very minimal and no stuck or dead pixels.
My eyes have to adjust to the AG coating. They aren't use to seeing a smooth picture that isn't "dirty".
I'm trying to pick the right trace...