I described both nVidia (NV40) and ATi (R420) speculation: NV40 as double an NV35, R420 as an R350 but with beefed up shaders. Really, current rumors are very thin on specifics, as both companies appear to be very tight-lipped about the new cards (probably because of their unusually intense current competition). Matrox is not and will probably never again be a high-end or even price competitive gaming solution, so it's not really worth considering.
Anyway, there's another thread above yours (posted by videoclone) discussing nV's and ATi's future chips. Check there for more info, what little there is.