Questions about 8GB DDR3- Please Help.

Hi, I think I am going to switch from a 780i to a 790i so I can use a sound card with my 3 GPUs, and because I found 4gb DDR3-1333 kits for just $150. But I have a few questions:

1) Where the hell is 8GB DDR3? I dont see a single kit on the entire internets. Is there something bad about 8GB DDR3 I should know about?

2) Assuming there's nothing terrible about it, will it work if I have 4 2gb ddr3-1333 kits? Will I still have 8gb and not see performance hits vs, say, 2 sticks of 4gb ddr3?

3) What would you say the performance gain from DDR2-800 to DDR3-1333 would be? I know DDR3 isnt worth it, and to be honest I dont want anything to do with it, but I'm going to need it so I can use a sound card.

Thanks.