on 2016 if will be of 80 GB at least an so on it will be huge on a point, now you could consider that you need at least a backup it does 80*2 160 GB + operating system + anothers personal data + music + photos + videos so you have your HD and uSb to limit, So how can it system be decentralized if almost of medium CPUS could dont be able of drive that charge. I dont can imagine if it get to massive adoption. Bitcoin XT look be a patch to support more transactions but for how long? Logically it never will be decentralized because almost peoples don´t will be able of storage entire blockchain and besides add use of more altcoins.
Its the dark side of crypto and digital systems--> The centralizations looks inminent The start? --> Cloud services. To unless that altcoins use solutions like to re-initialize the blockchains each year over certain time period.
http://www.ncix.com/category/hard-drives-8a-109.htm
Edit: It may not be very cost effective to buy anything under 1 TB