Here is a list of (mostly failed) combinations:
bitcoin-qt 0.9.6
512MB single core 1.6GHz AMD - forget it
1024MB quad core 1GHz raspberry pi - forget it. burned out a 64GB usb stick with too many rw before it had reindexed the blockchain
2048MB duel core 1.9GHz Dell - OK
6.2GB quad core 3.1 GHz Intel - OK, except that 0.9.6 is now obselete
bitcoin-qt 0.11.0
6.2GB quad core 3.1 GHz Intel - presently reindexing the blockchain (since 25th November)
in this situation, it's more safe to use a refurbished laptop PC with an i5 processor (like i5-560 ...).
200 USD for this setup on amazon.