For a fast Nvidia GPU with bootscreen on SL 10.6.8 I could not think of any models. What I know is that only the PC Nvidia 470 and 480, flashed with bootscreen can work with SL 10.6.8. There is the PNY Mac Edition Nvidia Quadro 4000 that also works on Snow Leopard but it may not be the fastest. Most of the fastest Nvidia drivers are on Mountain Lion or Mavericks.