I replaced the stock plastic (?) nut on my 50s reissue strat with a graphtech graphite nut. There's no difference at all in tone, and I haven't broken a string since fitting it.
There's some pics of the installation in the blog link in my sig. It wasn't too bad, but it did require a fair amount of sanding to get rid of the central notch that the nut came with. I think I just got lucky because it now seems perfectly fitted, and I definitely wasn't an expert (read: had never removed/fitted a nut before).
I'm not sure about the brass nuts. It sounds like they'd add a fair amount of weight, although that might be desirable - kinda like a permanently installed Fat Finger on the headstock :)
Hope that helps,
Jordan