Streaming Quality Improvement?

Hello to all

So I started taking the Streaming Video more seriously since that's all my children watch and to be honest I am the only one in the household that cares about quality, and the time that I can sit and enjoy a DVD or a Blu-ray is very limited.

So with that said, I am hoping to get great info here as the internet really doesn't have much information as to what I am seeking and the "Young" sales people at apple and bestbuy have no real solid answers.

So before I spend money on the wrong things, I come to you guys for answers.

So right now I am using:

Apple Mac Mini Server Late Model 2009 - Only used for streaming
Iomega Mini Max 1 TB - 800 FireWire
2 1080P Apple TVs
All connected Via Ethernet
All Ethernet Cables are Audioquest Cinnamon as well as the 800 Firewire Cable.

Now, from all of the research that I have done online, it comes down to internet speed is always the answer, but internet speed has nothing to do with me streaming itune Movies from the hard drive.

The video quality is good, but I've read reviews from people who say that theirs look close to Blu-ray quality and mine seems to look like a good DVD Quality at times a good S-VHS Quality. The dark scenes are horrible and there is pixilation problems as well no matter if its standard movies or HD movies. Now when I watch previews the quality is perfect.

So, can it be the Mac Mini's Video Card? Do video Cards matter when streaming? Is it the Hard Drive? Can it be that 800 Firewire is to weak to stream Movies?

Before I spend Money on a new computer and Hard drive I would like to pin point what it can be, because I don't really want to buy a new computer just for streaming movies.

Thanks in Advance, sorry for the long post

Enrique