You seem to be asking two questions, one about upgrading, the other about ES.
I can't say whether upgrading to the 965 is a good idea or not. I know that the higher 800/900 series chips tends to be a bad idea. Performance wise, they get passed by even the slowest of the C2D chips. If you can, get one of them. I can't really say what to get however, as I don't know what you have, or your budget.
As for ES, then can be like retail cpus, but you aren't even supposed to have an ES. ES chips are sent to motherboard manufactures so they can make motherboards that will work with them. Its illegal to even own them outside of whoever Intel/AMD has granted permission to use. This forum even had a nice thread in it awhile back where someone bought what was supposed to be a new C2D, but the company shipped him an ES. He contacted Intel with the information, and shipped them the ES, as its illegal for the company he bought it from to even own it. In short, don't worry about an ES, you probably will never see one. (it is possible to buy an OEM CPU, but OEM is different from ES. An OEM is exactly like a retail CPU, but it doesn't come with a heatsink or fan.)