As mentioned previously Games make a system successful.
NES destroyed all because it had a huge library.
SNES carried that banner.
PS1 took it, due to 3rd parties leaving Nintendo. Thus, they got the games and became the best selling.
PS2 carried that banner.
At this point it realy isn't clear. Presently it seems Wii has the best chance, however, even though I firmly believe that to be true, devs could drop it in favor of a high end multi-port scenario where PS3/360 would have the most games and thus may come back.
Basically, buy the system where the games you most dearly like reside. None of these systems are going to be like the Dreamcast. Sony has too much money for that to happen. Sega was already deeply in the hole when they launched the Dreamcast, Sony as a whole is nowhere near that.
