If anyone is going to buy an AC filter, please read the NuPrime Pure-AC4 product details. Even if you don't plan to spend that much on a high-end power conditioner, from the technical description of Pure-AC4, you can learn about that AC filter/conditioner.
Many cheap AC filter/conditioner (even some expensive ones) are just AC power strip with a fancy case and an ineffective filter.
Tripp Lite Isobar cost less than $100 on Amazon.com and it is very good with some noise filtering.