Good Luck with your new laptop. The specs look great. Now to your questions. Lets start with Gentoo. Gentoo is an awesome OS, but it is NOT for the faint of heart. There is no 'out of the box'. Everything has to be selected, built, config files created, modified, etc, etc..... What is offers is full control over every aspect of your system. ( the kernel config file has over 2000 items). It is not impossible, but has a long learning curve. That is where Sabayon comes in.
Sabayon, is an easy to install Gentoo base with their own modifications, many bleeding edge. What it has is a great installer, super graphics, all the tools / apps you need. I currently run Sabayon on this T41 laptop, everything I need working.
So, what you need to look at is are there linux drivers for the hardware in your Sager.
Video - nvidia driver 100.14.11 is installed. geforce 8600 driver is 100.14.9 so you are covered.
Wireless - intel 4965 is supported in Sabayon. see link http://sabayonlinux.org/forum/viewtopic.php?f=52&t=9194
WiredLan - I assume intel - should be no problem here.
Sata is supported
Intel dual core is no problem
1680x1050 resolution is supported.
The things you would need to check is what Bluetooth, fingerprint reader, video camera is used. Each of these has support ( depending on hardware used ) in Sabayon.
Looks like you have a great OS solution - Sabayon...
Hope this helped. Have fun