Let's concentrate on your streaming and playing media requirements for a moment.
The question that you need to ask yourself is: Will the mac mini act as the server, the client or both for playing media? I don't know how much you know about this sort of thing, but forget for a moment that your mac mini is called a mac mini server. It has nothing to do with what I am talking about. You've mentioned that you want to stream to a TV. I am going to try and cover some of the most common solutions, but this can be done in some many ways that I can't explain them all.
media client - the device that presents the media to you, allows you to search and select what you want to watch / listen to. This could be your mac mini, a Blu Ray player, an AV Receiver, a set top box such as boxee box, roku, Apple TV etc or it could even be your TV itself, if it has the functionality)
media server - the device that stores your media. This could be your Mac Mini or a NAS and so forth.
The basic flow is
media server --> media client --> TV
A media server and a media client are software, but they need hardware to run on. For example, boxee is a media client, like XBMC, but you can buy a boxee box which is set top box that connects to your TV and has the hardware to play the media as well as the boxee client software.
How you achieve this is very much up to you since you've a multitude of options. The simplest option would be to use the mac mini as the server and client. The mac mini stores all of your media and is connected to a TV via HDMI. You then run some client to be able to select and watch media (A client in this case could be iTunes, XBMC or even a rudimentary approach of Finder + VLC)
Mini -->(HDMI)-->TV
The problem with this set up is that you've mentioned this:
My usage Aperture and light Photoshop, light music and movie streaming to my TV. I want to have a central location for all my pictures, music, and shared data, and have remote access to my data.
Unless I am missing something, this suggests that you intend to use the Mini for Aperture and Photoshop? So then I have to assume that you're not planning to put the mini under your TV in the lounge and still attempt to work on it, unless you have an odd workspace setup? This means that you need a media client, if you're planning on using the mini as a normal computer as well.
Mini-->(Network)-->client-->(HDMI)-->TV
The network will be either wireless or wired Ethernet. I would recommend wired Ethernet if it's possible. Wireless streaming does not always work well, even with the best N wireless routers and how well it works will depend on where your TV, router and mini are placed and how many walls are between them. Wireless streaming is normally fine for SD content, but I run into issues when trying to stream 1080p HD content, even when the components are a couple of feet away from each other.
A lot modern TVs have a built in DLNA client. If you don't know about DLNA, then there is plenty of reading material on the web. Most NAS devices have a DLNA server. This seems like a perfect set up in theory. You can even get a DLNA server (software) that will run on your Mini.
Mini(with DLNA server) -->(Network)--> TV (with DLNA client)
Great? Well, not really. DLNA is wonky and the implementations differ from manufacturer to manufacturer so compatibility is an issue and it can be quite frustrating to get it to work. Even if you do get it to work then things like fast forwarding don't work very well and the in-built menu systems are usually quite poor and you won't get the media info scraping capabilities that you would get from using something like XBMC, boxee or Plex.
If you have a recent Samsung TV, then you can check if it has a Plex client. If it does, then you're in luck. You can get a Plex server to run on your mini and then stream straight to your Samsung TV. It will have beautiful menus and media info scraping, just like XBMC.
Mini(with Plex server) -->(Network)--> TV (with Plex client)
I am sure you can now see where this is going. There are different ways to stream media to your TV and whichever you choose should be the one that fits your current and future plans and needs best. Obviously going for a stable solution that's proven to work is a good idea as well.
But let's imagine that you don't have a TV with a plex client. You have several options
Mini(running iTunes) --> (Network) --> Apple TV --> (HDMI) --> TV
Mini (running network shares) --> (Network) --> Jailbroken Apple TV running XBMC --> (HDMI) --> TV
Mini (running Plex server) --> (network) --> Jailbroken Apple TV running Plex client --> (HDMI) --> TV
Mini (running network shares) --> (Network) --> Boxee Box or similar set top box --> (HDMI) --> TV
The first option will be the easiest and most seamless to set up. The other two options are a bit trickier and you have to get your hands a bit dirty, but you've mentioned that your media is in avi format. This is fine for most media clients, but it's not for iTunes. If you want to go with the first option then you will need to transcode your videos into an iTunes format. Handbrake will do this for you. It's easy to use and free.
In nearly all of the scenarios you could substitute the first device (mini) with a NAS, except for the running iTunes scenario. You cannot do this with any NAS right now to stream video. Some NAS devices, e.g. Synology, come with an iTunes server, but this is only good for music and Apple won't release the code to allow them to update it.
So you could have a set up like this
NAS (network shares) --> (Network) --> Mini (running XBMC or similar client) --> (HDMI) --> TV
NAS (network shares) --> (Network) --> Jailbroken Apple TV running XBMC --> (HDMI) --> TV
But this means that your mini will not be very usable as a normal computer for your editing/graphics work, unless you intend to work on the TV, which I wouldn't recommend, or have some long cables and an odd workspace setup.
I hope that this answers most of your questions, but let me know if you have any more.
My intended set up is actually this:
NAS (network shares) --> (Network) --> Mini (running XBMC) --> (HDMI) --> TV lounge
NAS (network shares) --> (Network) --> Mini (running XBMC) --> (optical) --> AV Receiver
NAS (network shares) --> (Network) --> Mini (also running iTunes) --> (Network) --> Apple TV --> TV Bedroom
NAS (network shares) --> (Network) --> Mini (running iTunes) --> (Network) --> Airplay speaker in kitchen
Here are some old screenshots when I was setting stuff up as inspiration. You just can't go wrong with this and even the Mrs approves
I will keep it on all the time connected to the TV in the living room via HDMI
