Which graphics card and to use hybrid SLI or not?

Hybrid SLi is pretty worthless.

Furthermore there is no point in getting either a 8400GS or 8500GT...both are incredibly weak cards.

For ~$75, you can get an ATI 4830 or Nvidia 9800GT which will perform significantly better than two weak cards in SLi. Or a 3870 or 9600GT for ~ $60