1) I'm hoping that's a typo and you actually have 3x2GB Kingston 1333 and 1x2GB (not 1x8GB) Supertech 1333 RAM. If it is a typo, the I wouldn't see the cost of upgrading to be worth the limited performance improvement you'd see. Stay with the DDR3-1333. If it isn't a typo and that is actually what you have, then I'd either consider getting one more 2GB Kingston DDR3-1333 RAM Module or 4x2GB DDR3-1600 RAM Modules; which ever you feel like spending money on.
2) The thread title mentions SLI, but I'm not seeing anything about it in your post. SLI requires the use of two identical NVidia Geforce cards. You cannot SLI a 560GTX TI with a Geforce 9800GTX.