Overclock speeds vary from card to card. The voltage on the core tends to affect the lifetime more then the speed itself. As long as your not getting driver crashes or artifacts you should be fine. Also just have to say, overclocking a GT 210 isn't going to get you very far. If you want more performance, even a $100 video card will far outperform that GT 210

As long as you don't change the GPU's voltage then any overclock you can achieve that is fully stable is going to be safe for the card. 63c is perfectly acceptable for a GPU as well.

It uses a 64bit memory bus so it probably would help a little bit, but I'm going to guess it's not going to be anything you'd notice outside of benchmarks. 

I don't know the exact numbers for a gt 210 but with video cards the core clock is normally more important. There tends to be a relationship where 10-15mhz increase is memory clock gives the same performance boost as overclocking the core 1-2 mhz. That's also in bench-marking, real world gains tend to be small. 
 
Good choice saving for a 750ti, those are quiet good budget cards. 

Yeah, unfortunately a GT 210 was the absolute bottom end card of it's lineup. The integrated graphics in a 4000-series Haswell processor would wipe the floor with it by an order of magnitude or two just to give some perspective.

Played Battlefield 3 on lowest settings and reso and got an average fps of at least 35. Before I used to only get a steady 26 but now it seems to be anywhere from 33-45. I think i'll be able to play BF3 and move on from NFSW lol :D