I have a Nvidia Gtx 770 MSI twin Frozr 2gb version and I have used Auto Detect in Watch Dogs but for some reason I get MASSIVE Fps drops when moving around the world. Like it comes to a complete halt sometimes. It is worse when it's driving. I tried turning down the setting all to medium and it still didn't fix the issue.
Since Watch Dogs is a poorly PC Port, there are no fixes for the issues you currently face. But since youre using an NVidia, you can download GEforce Experience and choose the Watch Dogs Profile, which will automatically set everything to optimized settings for the game. It wont remove all the issues, but it might make it more palyable.
I have the same issue with frame-drops with a GTX 760. I wasn't able to fix the issue, but I did alliviate it a lot by turning off V-Sync and setting "Max frames to buffer" to 2 (or more, though higher numbers caused noticable input-lag for me).
The screen tearing is terrible, but at least the game is playable.
I don't think the problem you're describing is due to your amount of RAM or your GPU as you/others have pointed out, but rather your RAM's clock speed. When moving around the world you're rendering the parts you're headed towards, if the RAM takes too long to load this it may result in these freezes/fps drops while moving, more notably while moving quickly, like in a vehicle. The only solution I could propose is to purchase RAM with a higher clock speed. This is of course assuming you currently like 8GB+ of RAM, otherwise I'd recommend purchasing more as @Elgert said.