Yes, It will bottleneck 4870
Mine is clocked at 3.2 Ghz and still bottlenecks my 4870, espatially on crysis warhead(this game is huge cpu demanded)
BUT, you'll notice GREAT performance boost anyway.
I've upgraded my 3870 to 4870 and noticed 50-60% performance boost, with better cpu it have to be more than 90% but I admit that cpu is bottlenecking my 4870. Can't wait for phenom 45mn release