Well if you want to have the entire blockchain which is currently 40+ GB, you will need around 50-55 GB for the full database, which will continue to grow at
relatively high speeds as bitcoin becomes more and more popular. The blocks will inevitably rise at some point in the future.
I recommend using prune mode which was implemented in bitcoin core no earlier than last summer.
Currently pruning the blockchain disables you from using Core as a wallet.
In the future this problem will be solved, for now you can rely on an SPV client.
In the post-prune era of bitcoin, bandwidth and memory will be the main issues, in contrast with the past where Drive space was.
Bandiwdth is crusial in order for sync quickly and propagate the blocks more effeciently, while
the RAM is responsible for holding the UTXO set (Unspent Transaction Output Set), required to validate transactions.
Currently the UTXO set is steadily rising, the spikes are related to the recent 'bitcoin stress tests'.
http://statoshi.info/dashboard/db/unspent-transaction-output-set?from=1420021203077&to=1442139603333You can see it is currently above 1 GB. Gavin Andersen observed that it roughly doubles every year.
At some point in the the future, 3 years from an 8 GB ram machine will not be able to even run the bitcoin core.
The truth is that point is even closer, as the RAM gets filled with other RAM hungry appications like the Browser and even the OS itself.
This is why I recommend at least 16 GB of RAM to be future proof.
So when considering the requirements to run a full node, RAM is more important.