the k8nxp-sli is what im using, its solid. you're limited to motherboards not because of who you bought from, but because of whats avalible. Currently the gbyte k8nxp-sli and asus a8v-sli are the only home user sli boards on the market (unless ive missed something). There is also the tyan sli motherboard, but it's made for dual opterons.
To answer your questions about the processors.... no.
The difference between the 3500 and the 3200 is so small you aren't going to notice the difference. You'll just be saving yourself 100$
If you're having someone else build your system, and you want good support checkout www.adkgaming.com
. It's where i work at, good support, good computers, good people. I've been working there like 4-5 months, they've been around roughly 8 years building pro audio, pro video, and sweet gaming boxes. If you're intrested or have anymore questions you can call up tomorrow 11am-7pm est and ask for tom (me).
fyi, im an hourly employee (im not self-promoting, i wasnt sure what the rules are here.)