Build my own new computer now, or wait for the new stuffs?

If you can wait it might not be a bad idea as things are transitioning to ddr4. Skylake is building on a platform with more pcie lanes than current z series. If you're concerned with power efficiency, that pretty much hangs it up for current amd anything. I understand you want a low power system but you also don't want to be choked when you need the performance either. It doesn't go both ways, you can't have a high end or fast machine on extremely low power. Even current k series cpu's don't use that much power when idling, only when you're actually doing something - and at that an intel based system uses half the power as an fx 8 core setup. More pcie lanes may be appealing if you wish to sli in the future and still need a couple extra lanes for a sound card or something else. Current z97 setups have only 16 lanes (skylake is supposed to have 20), running sli in x8/x8 occupies all available pci lanes.

No telling what skylake cooling solutions will require, they're not in hand yet. You don't have to pick 'energy efficient' or 'guns blazing', even an i7 4790k gives better performance at half the energy usage of an fx 8/9xxx. Easy enough to cool on air, even when oc'd. I don't see any need to go with an i7 's' series and end up with a crippled cpu for the tiny bit of power savings. Just my opinion on it. As it stands with comparable performance, nvidia cards are a bit more power efficient than their ati/amd counterparts. They tend to cost more as well though that could change if you have to buy a higher wattage more expensive psu to run the cheaper amd card.

Many times amd gets the bang for the buck vote in that it does well enough for a few less dollars. Keep in mind any performance trade off vs cost difference spread over the length of time you plan to keep the machine. If it's 3yrs (your target), keep in mind if saving $60-100 now is important over the course of 3yrs. At the lower budget amd does have the bang for the buck vote many times, problem is amd quickly runs out of 'bang' and no amount of 'bucks' is going to change that.

If you're not a diehard gamer, a titan x is a waste of money. It's not going to improve the rest of the things you mentioned, multimedia (an igpu would do this), photoshop (very little benefit over an older mid range gpu and only some features use the gpu), audio etc. Since you mentioned running multiple applications, focus on 16gb of ram as the baseline. Single core performance is definitely important, it comes into play with everything you do on a pc. It's not a performance metric that only comes into play running a single program, it's the heart of the machine's capability. Exactly the reason why so many times amd's 8 core cpu's are outperformed by intel's quad cores. Less cores but more powerful cores still get the job done faster.

If you're unsure, pick some of the programs you think you'll be using and look at benchmarks for those programs. One of the biggest misconceptions are synthetic benchmarks such as passmark scores. Amd often times consistently scores high on synthetics, but if it was a reliable way to measure performance you'd think it would translate to real world scenarios and usually it's just the opposite. Synthetic scores don't make real day to day programs run any faster.

Unless you're in dire need of an immediate system, it may be worth waiting for the newer things to come out since it's close to transitioning (new cpu's, new motherboards, new ram, new gpu's).