Here is some information I wish had been available when I upgraded my system recently.
Yes, the Gigabyte GA-K8N Pro-SLI motherboard is supported by Linux!
-sata_nv driver for SATA
-Network: nVidia CK804/Cicada CIS8201, use forcedeth
-Audio: nVidia CK804/Realtek ALC850, use snd-intel8x0
-USB, use ohci-hcd and ehci-hcd
-IEEE 1394 Texas Instruments TSB82AA2 IEEE-1394b, use ohci1394
-i2c_nforce2 for nForce4 I2C bus
-ITE IT8712F sensor chip, use it87 driver in Hardware Monitoring section
-infrared: unsure, not using
These drivers should all be available in stock kernels from major distributions (I think). This means Ubuntu and Fedora and stuff probably work. I use a custom kernel, and here is where to find the drivers in the kernel config menus.
-Power management options
-CPU Frequency scaling
-AMD Opteron/Athlon64 PowerNow!
-AMX and nVidia IDE support (unsure if this is necessary)
-AMD CS5535 chipset support (unsure if this is necessary)
-SCSI low-level drivers
-NVIDIA SATA support
-IEEE 1394 (FireWire) support
-Network device support
-Ethernet (10 or 100Mbit)
-Reverse Engineered nForce Ethernet support (EXPERIMENTAL)
-I2C Hardware Bus support
-Nvidia nForce2, nForce3, and nForce4
-Hardware Monitoring support
-ITE IT87xx and compatibles
-Advanced Linux Sound Architecture
-Intel/SiS/nVidia/AMD/ALi AC97 Controller
-EHCI HCD (USB 2.0) support
-OHCI HCD support
Even though the network driver is in the 10/100 section, I have read that it supports gigabit.
Athlon64 X2 Warning
I recommend enabling Smart Fan Control in the BIOS. It will make the cpu fan quieter.