My understanding is you can do a lot with PS itself, but I don't have any experience with that. But if you want more, I'm 99% sure you can use Silver Efex Pro with PS alone and I absolutely LOVE Silver Efex Pro. Topaz has a B&W plug in that's pretty well thought of, but I think Silver Efex is sort of the gold standard...
I used both SEP and Topaz B&W and 9 out of 10 times, its SEP that I choose. That said, Topaz B&W Effects has just as many available options where parameter setting is concerned, and a lot more presets to fiddle with.
Another thumbs up for SilverEfex. In fact the whole Niks Suite. I find myself using ColorEfex, Define (for noise control) and SharpenerPro quite often. Viveza is a good tool as well, especially for creating quick edits using luminosity masks, but one can have more precise control in PS, even from a single image (http://goodlight.us/writing/luminositymasks/luminositymasks-1.html).
Of course $150 for Silver Efex also buys you the entire Nik software suite. There is also a 15 day free trial for it as well. Assuming that you want to invest the time in processing, a software bundle like this gives you more bang for your buck than a camera upgrade.
Its worth the money for the Nik suite. We all jumped on it, I think, after Google dropped its price from the stratospheric $499 that Nik was charging for it. Up until then, it was (koffkoff) "evaluation copy" for me.
The NIK suite is really excellent value - a suite of great plugins that play very nicely together. Particularly for people like me who do want local adustments but are too lazy to sit there brushing things, it's an ideal package. I prefer using it in Photoshop due to the fact you get a new layer per adjustment/plugin you use..
I also have SEP and like it a lot. If I had/liked Mac products I would probably have Macphuns Tonality Pro. That program looks pretty sweet. But here lately since getting OnOne's Perfect Effects Suite I've found myself using their B&W quite a bit. So that's another one to look into if you want to.