I would sell the computer as a whole. These are pretty old parts so not many people are looking to upgrade into these specs (besides 6970). Just advertise it as a gaming system and some noob will buy it for more than you can sell them separately. Make sure to advertise what games it can play and at what settings.
If you are selling the monitors as well, sell those separately. Sell the speakers separately as well if you don't plan on keeping those.
Lastly i wouldn't recommend getting a gtx titan. Very expensive. 2 gtx 670's would be a better buy. Heck even a gtx 690 is a better buy.