I absolutely agree with using Ghostery (or something like it) for privacy reasons. That said Ghostery and AdBlock both use quite a bit of memory*, and IMHO slow things down as much as the ads they are blocking (apart from flash ads which FlashBlock or native click-to-play capability solves with much less overhead). Furthermore, I almost never see ads when running Ghostery, and conversely the EasyPrivacy filter list for AdBlock does much of the same thing that Ghostery does. So I would recommend trying them both out, and then sticking to just one rather than running both at once. Also, if you use Ghostery make sure it is configured to block new elements by default.
Lastly, if you (or someone you do tech support for) refuses to use Ghostery (or NoScript) because it sometimes break webpage functionality, Disconnect is a good option to look at. It doesn't block nearly as much as Ghostery, and isn't as informative about what it is (and isn't) blocking, but it is better than nothing. I have never had it break a website, and requires no config tweaking.
*Note, the memory usage issue may get better in a couple releases.