SLI ready in AM3 hasn't been in the market just yet, they have built a motherboard that does have sli format for $200. Just have to wait a while longer for it to hit the stores for sale. DDR3 1600 2x2gb will do just fine, triple channel is more along i7.
For 955, depending on your budget, Theres a sticky that TheAnimal has posted on the beginning page of Homebuilt systems. Please copy and repost it in here to help us, help you out. Thank you.