The system that I'm using the Abbey Road Plug-ins TG 12414 Mastering Filter (and the full Mastering Pack bundle) with is a Pro Tools LE based system. I'm running everything on a Mac Book Pro computer, made up of a 2.2 Ghz Intel Core 2 Duo processor, and 4 GB of RAM. I run Pro Tools with a Digi 002R audio interface and I monitor with a set of ADAM A7's. This has definitely been enough to support running this plug-in, and I've used it both for mixing and mastering. It's doesn't seem like the type of mastering plug-in that takes up too much of your system, or at least it hasn't on mine.
The Abbey Road Plug-ins TG 12414 Mastering Filter is a great overall equalizer plug-in for both mixing and mastering. While it's dubbed a mastering plug-in, and it certainly is, I've also found it to be great for mixing as well. It's not the most flexible plug-in, but for a one band essentially, there's definitely a lot that you can do with it. I think it has a great overall sound, as it's easy to add some grit or some brilliance with just using the single band. It's also nice that you can easily drop low or high frequencies off that you don't want or that are unnecessary. The Mastering Pack is just made up of this and the other aforementioned TG 12412. It's definitely best suited as a mastering plug-in and bundle, but it's more than nice that it's versatile. Abbey Road makes some great plug-ins that sound quite realistic - the TG 12414 and the Mastering Pack as a whole is absolutely worth checking out if you're in the market for something like this.