Will be looking at Nvidea's 560Ti series or ATI's 6750 series.
AFAIK HD6750 is only available to OEM and they we direct re-brand from 5750 with minor improvement. btw regarding your question i think it is fine to overclock your gpu or cpu. current hardware are more tolerant to overclock compared to hardware from past years. in fact overclocking become one of selling point these days. if not we won't be seeing k series cpu from intel. regarding the life-span the OC might shorten it but i know some people OC their CPU for 2-3 years and they still fine. however cooling is important when you overclock since the core (gpu or cpu) will generate more heat since they operate on higher frequencies than they were rated. if you not sure how to overclock it is better for to look up on some guides before making an attempt on yours especially on the overvolt matter. overvolt are needed if you want to go higher than the stock voltage can allow but if done wrongly it will fried your component.