Duo core e6600 & e6700 - is there a difference apart from price? - you decide: see this 18-page review with benchmarks click here. I'd go for the E6600 as I don't think the E6700's inflated price is worth the relatively small performance increase.
nVIDIA SLi - is it worth it? I'd say no, especially when you're already getting a good enough card. You can always upgrade to SLI if you want/need to in the future - the same card will be less dear too. Check if the motherboard is SLI-enabled.
Add another DVD-rw drive as a backup drive & for multitasking(?). They're inexpensive nowadays, even the new 18x models.
I'd go in for another hard drive (external if you can afford it) for backup/archiving purposes, & also a 'better' power supply - Tagan, OCZ, Antec are some good brands. Good luck with it all : )), G