Yes, however instead of a GTX260 I'd recommend an HD5770. They have the same performance, yet the HD5770 costs about $55 less, and has better technology, as well as using less power, outputting less heat, etc.
The latest GPU you can get is the radeon HD5xxx series, which includes the 5770. Its the latest with DX11 and Eyefinity, and is similar performance to the GTX260 for much less $$. I'm not sure where the bottlenecking would start... you might get away with a 5830, but I'm doubting you could go too much past that.
Yeah- a bit of an OC (3.1-3.4 like Shadow said) would let you run a 5770 pretty solidly without bottlenecking. Going to a 5850 or 5870 would probably want a bit more CPU power to really stretch their legs and run full out.
Bottleneck - the most feared non-issue of mid range gaming. I used to worry about bottleneck issues with my e6750, too. Trust me, you wouldn't have to worry about bottlenecking at the slightest as long as you crank the CPU to 3.0+ But ask the same question again in one or two years and answers will be vastly different. For now, don't worry about it.
You never HAVE to OC, but if you want to get the full potential out of a 5870, or possibly even a 5850, you will need to. a 5770 or 5830 would probably be a better straight up fit. An OC will be very useful though in all situations at this point as it will make sure you can use your GPU and have better desktop performance to boot:-)