You only need one SSD for the OS if boot time is a worry. Physical HDDs are plenty fast enough for movie playback. Also, if you build a dedicated HTPC, it means that you will not need to load 95% of the crap that usually gets placed on a typical home PC. You would be very pleasantly surprised how fast Windows 7 boots up with a HDD PC that doesn't have a ton of crap to load at boot up. I know I was.
Still, it's admittedly a luxury item that your probably don't realllllly need so I won't try to talk you into it. However, if you or others are like me and like to do stuff 'just because' then here is a bit from my experience over Christmas vacarion. I can post full parts lists later if you like.
You can build a HTPC for a very reasonable price now. Even a dual core based machine will playback video nicely but more power and memory is always going to be a bonus if it will also used to rip your content. Getting a Bluray compatible graphics card is not expensive anymore but you do have to do some research into models to ensure things like HTCP compatibility. Cabling straight from the HTPC to your AVR solves all of your streaming quirks and issues. It can require some tweaks and an understanding of formats and resolutions to get it just right but once it's configured to match your TV, Projector etc. It will stay seamless.
I'm using XBMC (free) as the media front end. There may be better choices for larger music/video/picture selections but it seems to suite me very well and I like that it is open source so there are tons of plugins to make it look the way you want it and to make it more useful than just a movie picker. This includes the obvious like Youtube, Netflix and a web interface that allows you to use you iPads, smart-phones and other tablets to control it if you wish. Lots of other plugin options to popular web interfaces and music/video sources as well. It has a large following so I expect it will stay nicely updated.
You can also get HTPC designed cases that actually look like AV gear instead of a big boxy PC and they will often have quieter cooling so it plays nice on your AV rack. I went with this one. Antec Fusion Remote Black
I knew from the start that I probably would not find the display or the onboard volume control useful. In fact, I disabled the display as it was too bright. However, it was the fact that it looks like a piece of AV gear that gave it the required WAF. The inside was very well designed. Room for everything in well though out positions with adjustable bulkheads to keep the cooling flowing the the way it should.
IO Gear makes some nice, small footprint wireless keyboards with a built in trackball that won't dominate your coffee table but are still very usable. I am happy with this one. IOGear HTPC multimedia keyboard with trackball and scroll wheel.
My planned budget was for $600 and I could have made that work. However, I inherited a nicer motherboard and CPU than I might have chosen and some RAM. If I did a total of what the entire box would be worth if I had actually purchased everything, I would have just broken the 1k mark.
It was a fun project and I'm happy with the results.