Is this an okay overclock for ASUS STRIX GTX 970 (GPU Tweak II)?

Looks about right.

Just a note, you won't have any control over voltage on that card unless you hardware mod it. So you can only use the stock voltage. Pretty much every Maxwell card has a maximum default voltage of 1.212V (mine 980 is the same), the actual voltage is usually a little less. How much less is unknown unless you want to measure it with a DMM.

Your biggest enemy here will be heat. Keep an eye on it, so far your temps are pretty good. If you aren't already using a custom fan profile, you may want to look into that.

As for overclocking, it's always better to find the maximum overclock for each clock independently. Usually the core clock is going to give you the most bang for the MHz. Put the Power Limit to 125% which is the maximum allowed. So start by raising your core clock with the memory clock at default. Test every time you raise it. Something like 3DMark or Unigine will work. Keep raising it until you experience any type of instability. You may not, the card will lower it's clock to stay within the power and temp windows. So if you see the core clock throttling down, you've hit the limit of what your card can do. This can happen even though you aren't experiencing any instability. nVidia was pretty conservative with PowerTune.

Once you've found your max core clock, note that frequency and then return it to default. Then start on your memory clock. Same procedure here. In all likelihood, you will get to a point where you experience problems as raising the memory clock doesn't affect power usage and temperature very much. So here you may start to experience artifacting or games crashes, crashes to desktop, driver unresponsive messages. Once you've found the maximum mem clock return everything to default.

Bringing it all together. It's unlikely that you will be able to just set both to the maximums that you've found. So firstly, like I said the core clock is the more important of the two. So set the core clock to the maximum you found. Then set the memory clock for about 66 - 75% of the maximum overclock you found for the memory and test. If it's stable, keep creeping the memory clock up until you start having problems, or PowerTune starts down clocking the core clock. You will probably see PowerTune start down clocking the core clock. Even though I said memory overclocking doesn't affect the power usage much, it does contribute, so if the power limit is reached, PowerTune will lower the core clock. This is something you don't want, so just below that point is where you'll want to leave your memory clock.

Hope that helps.