Upgrade to CS5 and Acr 6.x. You won't need a plug-in.
Wo Tai Lao Le
I'll put my name under that any day. ACR 6.x noise reduction is a monumental improvement over 5.x (and vastly better than anything in Photoshop). I now find I can shoot ISO 1250 and get the same quality I used to with 400.
You can also set defaults specific to ISO setting, which of course is particularly useful for noise reduction.
I'll second (actually it looks like I am thirding it) the nr in ACR 6. I have never managed to get into Lightroom (it's paid for and on my system - I just never use it), so if I have a large job to process I always use the nr in ACR because it does a perfectly OK job, and is QUICK. Otherwise I use the Topaz deNoise plugin, but waiting for that on 200 to 300 images is painful, and I am not convinced it does a better job than ACR 6 anyway.
I don't often agree with Tai Lao, but in this case he's absolutely right.