When I record a 2.5 hour movie to MPG format on my TV tuner it takes up about 5GB of space. I Have found that my 500GB hdd is loosing space very quickly.
If you manage to get them compressed onto MPG format and fit under 5gb of space
2 500GB HDD is not unreasonable in your case. Also I would not recommend streaming video wireless unless you got a wireless N connection.
So if you manage to get them compressed onto a MPG using a on card TVtuner compression then you could do it with 2 500GB HDD or a Terabyte Hard Drive.
I would not recommend Windows Midea Center Edition becouse then what pip22 Says about 13GB is completely correct.
What I use for my PVR/Media Server/File Server is MythDora Plus Best of all it is free. Here are some instructions on how to install it. Here is where you get it
If you want to Get a Tv tuner to add to it I would recommend Getting a Hampague TV Tuner. They seam to work the best and in addition to that they encode on the card so it takes up Less CPU.
I would also invest in a good IR computer Remote one of the sides above did a review on one that I am planning on getting.
Also for redundancy might want to think about Doing a at Least A Raid 1.
Don't ask me how to do it yet that is my next project is setting up a Raid. Might want to go to the Hardware Forum for more info on that.