The HP Home Server is what I use and it is much smaller than you would first imagine. It is not much larger than the four drives it is designed to hold. It also runs fairly cool. I bought the 500GB model and added two 750GB and a single 1GB drive.
After three months of use I have observed:
It streams wireless to my XBox360 (using media center) with very few difficulties with music and SD video through two walls-60ft.
It is not the fastest drive to upload to that you will ever find, but it is tolerable. Mine averages about 5 minutes for 30min of video. It can get a bit cranky when left on its own (about once a week it has to be rebooted, but I think it is a problem with my ISP changing settings).
Overall, I recommend it for digital storage with the disclaimer that I have not attempted to stream HD video from it yet.