Is this a good OC for this gpu

I'm not familiar with that card at all, but you may be able to squeeze a little more out of the memory if you raise the target power level. I run +45% on the target power level and can get an extra 520mhz out of my memory on a pair of GTX 670ftw.

 
Well.....like you said, it's a GT640. Push it is far as you can. Raising the power target does nothing but allow it to draw more power to help keep it stable, and maybe get a bit more out of it, without bumping up voltage any more. Raising the clock on the memory shouldn't add that much extra heat, but could help overall performance of the card.

I see no downside in pushing it further.... It's a low end card on 2 year old tech. If ya melt it, spend $150 on a 750ti and end up with better performance at stock clocks with lower power draw (Maxwell architecture), than you got out of an overclocked 640. Just my two copper.