I'm running the Antares Systems Avox Duo in Pro Tools LE 8 currently on a Mac Book Pro that has a 2.2 Ghz Intel Core 2 Duo processor and 4 GB of RAM. I run Pro Tools LE with a Digi 002R audio interface or a Digidesign Micro Box if I'm on the go and I monitor everything with a set of Adam A7 monitors. I've never had a need to run more than one of these at a time in Pro Tools, so for what I've used it for I've never had a problem with the performance of this plug-in.
While definitely a cool idea, I'm not sure that the execution on the Antares Systems Avox Duo is the best. It sounds decent enough, but it's definitely not a very realistic sounding vocal doubler and I wouldn't print with it for any real mixes. The tracking isn't the best on it, but it's decent enough if you're just looking to add a little bit of depth. Honestly, if you want a vocal double I don't see why you wouldn't use add a second voice to your recording. This is a quick way to get a double vocal in there, but again, I wouldn't use it in a serious mix. I'd recommend trying out the Evo Duo if you're seriously interested in the plug-in like this, as that one sounds a bit better. If you're interested in the other plug-ins in the Avox Toolkit bundle, it's at least nice to have this one, but I wouldn't get this plug-in pack for this plug-in alone.