I started out much like you. I had some movies on my PC that I wanted to play on my TV, and I also wanted to stream some stuff from my PC on my TV too. I got into HTPC for this reason. Quickly I wanted to do more, like media organization, and use a slick front end like MediaBrowser or XBMC- and soon I need more storage space to hold my stuff. Originally I used one machine to to it all- My desktop. It was a good machine, i7 with SSD and I had lots of storage in it. I simply ran a long HDMI cable under the floor and to my AVR receiver and I lived happy that way for a few months. I wanted to watch on two TV's (my 55" Plasma in living room, and also my projector in my theater room) so I run two HDMI cables- each from my AVR to each display.
I was noob and just learning and I thought this was want I wanted. But I soon realized it was cumbersome, unreliable, and a general hassle. I wanted something with more storage- and also more appliance like so I can use a remote control, and my wife could understand how to use it.
So I build a cheap HTPC for about $250 and I installed that into my theater system. I hooked it up with HDMI to my AVR. It was SSD only machine, so it was quiet and cool. I networked it into my desktop with Cat5e cable I already had run years earlier, and the desktop served as the "server" and had the storage hard drives in it. That seems to work better having a dedicated HTPC that is appliance like installed on the actual TV system rather than use my PC for it. But there was still a major issue, If I restarted my PC or it was not on then the HTPC would not work. I was successful in getting my wife into the HTPC and since I was a PC geek and spent a lot of time on my desktop every time I restarted, or crashed- or messed around with my desktop the HTPC would stop working and she would yell at me.
It was obvious I needed a dedicated, reliable, always on media server. I also needed more storage than I was able to hold in my PC.
After building a media server- it's the best thing I ever did. It's very reliable, very upgradeable and expandable, very affordable, and it works wonderfully without any fuss. I never need to restart it, or install anything. It's truly stable and very much set it and forget it. My server holds our pictures, our media- it provides automated backup of my HTPC and laptop and desktop. I can access it from anywhere- and it run PLEX so I can watch my stuff anywhere I want over the internet ( you can soon do this with mediabrowser too) PLEX is just like XBMC but it allows for transcoding and uploading over the internet to mobile devices like laptops, cell phones and tablets when your away from home.
Having tried to do all in one- and then living with a dedicated server and dedicated HTPC I can say without any hesitation that all in one is not a good solution at all.
I think what you want is a server for storage - and some client HTPC's on your displays. At least one, possibly two if you want two independent systems on two different TV's. Your better off splitting up the duties and making the server to handle storage and back up, and making the client HTPC's to handle streaming, playback, running your front end etc... You do not need or want to hook up your server to your TV. You can let is just sit someplace like a closet.