CS4 will have installed the 32-bit and 64-bit versions automatically. I use the 64-bit version of Vista. (12GB's of triple channel RAM is fun!)
NIK plugins will not work (yet) in the 64-bit version. I have been in contact with them on several occassions and they have something in the works but nothing yet. Same thing for OnOne plugins...
That said, most of Topaz Labs plugins (I own them all) work well and are written for the 64-bit version. They are fast and stable. Neat Image 64-bit is great, VERY fast. There are a number of other plugins that work in 64-bit mode...it's just a matter of time for them all to catch up.
Yes, I am a plugin junkie...by the way, it's not a plugin but if you enjoy doing panoramic stitching check out Autopano Pro2. They have a 64-bit version and it is absolutely smokin' fast, it also can utilize the GPS like CS4.