A custom water loop would allow you the best overclocking potential. This is simply because water has far better heat transfer properties than air - which allows it to carry heat away from your CPU and out through the radiator. For example, you can easily get 5+ Ghz on a chip with water cooling as compared to maybe 4-4.5Ghz on air.
- Highest overclocking potential
- Lowest CPU temperature
- Can be practically silent
- Can be expanded to cover other components later on (like GPU)
- Looks cool
- Can get very expensive very fast
- Maintenance definitely required
- Not easy to set up for beginners
- Risk of damaged components due to improper installation
- Swapping out components might not be as quick and easy
Dual Titans would definitely give you better performance than triple 680s. Reason for this, apart from the obvious lack of power on a 680 as compared to a Titan, is that SLI scaling for 3 GPUs is pretty bad. You're not looking at 3x the performance of a 680, more along the lines of 2.5 at best. Dual SLI scaling, on the other hand, is much better than it was a couple years ago. You would usually get the full 2x scaling with two cards, or very close to that. The general rule of thumb is to buy the fastest (most expensive) GPU you can afford, then look into SLI later on. Its never a good idea to go with multiple cards (I made that mistake and I regret it) without first getting the fastest single GPU.