bsmegreg
First off congrats for winning. The GPU is going to be dependent on your use of the system.

950- Spreadsheets/workstation/low end gaming.
960- Can do 1080p gaming, but not good (low budget).
970- Ideal for 1080p 60hz gaming.
970SLI- Ideal for 1080p 120/144hz gaming.
980- Not good for its price, go 980ti instead. It's basically a 970, but that's off topic...
980ti- Top of the line, ideal for 4K or 1440p. Overkill for 1080p, UNLESS you're going 120/144hz.
980ti- Better than one. Only really worth it if you want 4K or 1440p at 120/144hz.
TITANX/TITANZ- Not worth it, basically a 980ti for twice as much money. It's technically better, but barely.

If you have any more questions, just ask.

Makalaure
I ran 970s in SLI (4k GSYNC) and then "upgraded" to a single 980 Ti. On paper, the SLI 970s outperformed the 980 Ti, and in synthetic tests, there was no contest, the 970s smashed the Ti hands down (and consistently earned me approx 60% more folding points/day on average). However, SLI brings it's own problems, and in gaming I would say the single Ti is smoother, although I do lose approx. 5-6fps on average. The exception was Shadows of Mordor, which just ate through VRAM like there was no tomorrow, and stuttered horribly when on Ultra. I imagine any games that consumer 4gb of VRAM will likely fair the same, but a tweak of the settings can resolve this.
 
So, in all honesty, in terms of real world applications, I prefer the 980 Ti, but in terms of synthetic benching (and folding, for that matter) the 970's destroy it. Funnily enough, I bought one 970 cos that was my budget. I saved up for another one a few months later, used them both in SLI for a month or so, and then sold the pair and bought a Ti with the funds, so I think I made out like a bandit.