As above, the only AM3 SLI boards use Nvidia chipsets and have not been made for a few years.
Instead of getting one of these boards (and you need more GPU power) then you are probably better off either:
- changing your GPU to an AMD one (use a crossfireX capable AM3 board if you want to be able to add a second card in the future)
- change your GPU to a much faster single card
- change your motherboard and CPU to Intel X58, P55 or P67 - certian boards using these chipsets properly implement SLI