I already know that Softimage|XSI is going to be my primary modeling tool, but I've been thinking about a detailing tool. I've become aware of these three:
Zbrush
Mudbox
3D-Coat
I've tried the trial of Zbrush, and from what I can tell it is a very unlimited program with an extremely easy interface. The video on pixologic.com with all the artists talking about it indicates that it can be used to make entire characters from scratch as the primary tool, and with some texture work in an image editor, a polygon-reduction and a normal map computation, a game-ready model is right there. But I'm still wondering if I need this kind of functionality in something I'll mostly be using for simple detail painting and texture prototyping. It costs $600 for the latest version, meaning just a bit more saving and no money for other tools.
I haven't tried any trial of Mudbox, but from what I can tell it does what I need it to do just as well as Zbrush. I don't know much about it, but from the timelapse video I saw of one artist making a head from a box, it will work just fine. I don't know the price, and I am not familiar with the interface so I can't give a thumbs-up or a thumbs-down on that.
3D-Coat looks like exactly the kind of thing that I have been pondering about for a while. A UV-Mapper, texture painter and detailer all in one. It does exactly what I need it to, plus UV-mapping, and it's only $120, leaving the larger portion of my budget for a few more tools. Still haven't tried the demo yet, looks simple enough, cheap and effective. I may very well get this, especially if I decide Zbrush is too professional and pricey right now, and if Mudbox doesn't beat it by both of those issues.

Remember those old guys? They made epic renders, I think one of them was called DaVinci, and all they used was MS Paint. Sometimes it's just skill....