I am using Topaz Denoise with CS5 on a MacBook Pro.....love it, easy to use and does a good job...not sure if it's 64 bit or not.

I also received Nik Define as a gift for Christmas but have not used it much, it has an interestingfunction where it can be set to automatically analyze various parts of an image and selectively apply noise reduction. It seems to work well but I have used the Topaz plugin so much that I haven't explored anything else extensively.