Recommendations on $300 to $500 Intel HTPC Build

Your parts look good.
I might suggest windows 7 home premium since it already includes the media center application.

A media center remote should not be expensive.
This M-ATX B75 based motherboard should do. http/www.newegg.com/Product/Product.aspx?Item=N82E16813157356
Here is a low profile 7750 graphics card which will do fair gaming:
http/www.newegg.com/Product/Product.aspx?Item=N82E16814202002

I think home-built HTPC's are very problematic,in terms of control, noise, cost and size but if you're convinced go ahead. Rather than give a parts list I'll simply list a few points.

I was going to build an HTPC years ago but went a different route. Even today I wouldn't build an HTPC.

A few points:
1) The ASUS OPLAY and WD LIVE boxes are really great media-only devices. They have the advantage of simplicity and remote control.

My WDTV LIVE can play absolutely every video I put on the hard drive and it also supports NETFLIX etc. I recommend the CHEAPER version without a hard drive then buy a 2TB 3.5" WD Elements instead.

2) For $80 to $100 the standalone media players can't game or even browse (at least well), but might be a good idea for at least one your TV's.
*I know you can STREAM from a PC to my WDTV LIVE.

3) A LAPTOP might be a good choice for a HTPC. Small, quiet and HDMI just works (you can have issues with building your own PC). The only drawback is gaming performance.

*Something like THIS is achievable I believe today:
1) Turn on laptop via remote control.
2) Boots to 3rd party MEDIA CENTER by default.
3) Navigate to video etc.
(I'm not really up on how well you can navigate with a remote control. Can you watch a video, then open a Browser without a mouse/scroll pad?)

STEAM is making a lot of progress with control via the XBOX controller even for Web browsing but it's nowhere perfect (works within Steam in "Big Picture" mode).

4) MEDIA CENTER for Windows sucks big time. There are other better solutions though.

5) PS4/XBOX ONE are other interesting choices for gaming/web browsing. They are obviously designed for your HDTV.
*Any gaming PC you build is going to be far more expensive and far noisier than a PS4 (for similar gaming experience).

I spent a lot of time optimizing fan noise for my gaming PC but I still wouldn't want it in my living room. Sure, it wouldn't be as noisy with less-demanding parts but if I was going to do that I'd just get a PS4.

I'm considering the PS4 for my living room even though I have a good gaming PC; it's also for company as well. I have lots of media (cough) such as Anime so I'm waiting to see how good media support is (especially MKV container support. Probably a no-go though.)

My other interest in the PS4 is how well the web browser works. I think the PS4 and XBOX ONE will put a little effort into making that work well with the controller (or for the XBONE you can just yell at it "XBOX. Go. To. Porn." Hopefully you don't then accidentally video Skype your mom.)

PS4 features I'm interested in (besides gaming):
1) Multimedia support
2) USB hard drive media support?
3) BluRay
4) Web Browsing.
5) Netflix

Out of the above it's probably only the Multimedia codec/container support that concerns me.