Why do you think you have a CPU bottleneck? Unless your playing some serious RTS's or something I doubt there would be much of a bottleneck with a GTS 250.
If you really want to OC then get a better cooler, I'd suggest the CM Hyper 212+ if your going for one. You are not going to get far if you are already on 50 C temperatures. What did you load it with? Prime 95? I reckon maybe to 3.2 or a bit higher while staying under 65C.
Also you could take your side panel off and check your temps to see how bad your airflow is.