If you can post the exact model number/type of the PC you have, it'd make things easier. Most PCs have a power supply unit that is upgrade-able, you just buy a new one and pop it in and reconnect all the cables to their specific points, but some brand name/propreitary systems can be a bit of a PITA when it comes to putting in new PSUs, others here will sound off about that I'm sure once you post the computer model number/etc.
If it's an easy swap, there are piles of 500w PSUs out there for 50 or 60$, the best ones would cost closer to 80$, but you can get a perfectly decent gold rated 500w PSU for 50$ right now from a number of manufacturers, more than sufficient for what you're after.
We've got a 34" 1440p 100hz lcd on one of our systems now too, you'll probably be fine with either the 1070 or 1080, but IMO pushing that much screen at 1440p, you'll be better off with a 1080, and if you have to upgrade the PSU for that, 50$ is NBD I'm sure. You might get away with the one you've got, as the 970 is supposed to require a 500w PSU too, but I'd like to see some tests run on your current system right now to see if the 970 is being throttled by lack of power, it wouldn't surprise me if your card right now isn't running to its full potential due to being starved for power, but it also wouldn't surprise me if it's running as normal too. Hard to say without checking.