not much you can do to future proof it, within a couple of years the standard will probably be 5-10Tb. Just make sure it does what you need out of the box. That is it should support media streaming etc.
Two decisions you need to make.
1. Do you want to make sure the data isn't lost? if so you want a box containing at least 3 x 500Gb disks and supporting RAID 5. In which case go for something like a Buffalo Terastation.
2. Do you want to be able to stream media to internet radios, music players etc without having to run a PC in the background? if so you want something like a
QNAP TS-109.
I have one of each of the above (the Buffalo being the older model without media streaming).
You can, of course,
get models which support both, but the price goes up.