Well a 780 or a 690 is a better solo card, right? But yes, I would just build a rig that is at the top of what you would need today and then upgrade it every couple of years. You'd get as much power as you need, much way better value and it would last way longer.
That's because they're dual gpu's, and take up two expansion slots
The titan has the fastest existing single gpu, and it can fit into just one expansion slot