I actually have the same cpu and started overclocking yesterday. Yeah, 70-73 is ok as long as you don't keep that temperature for like 5 hours straight. I'm planning on getting a cooler even though I plan on upgrading in the near future but you can use the cooler on the new cpu too, it will just keep your cpu alive for longer and you can overclock more, I've managed to overclock up to 3.78Ghz and stay completely stable but it causes your cpu to go up to 85 degrees at times. Definitely need a cpu cooler
GuysWatch this video. This CPu is far one the bet to overclock watch my video and learn.
I am a holder o a intel core E5400 and I ave proof of the overclocking capabilities of this chip, y should watch my youtube video and buy yourself a E5400 CPU for yoour next build look at this and convince yourself: http://www.youtube.com/watch?v=thSBjWshsb0