BIOS update: check the motherboard manufacturer for instructions, it can be a dangerous process.
Thermal kexts: some *.kext files installed with Leo that control CPU temperature, I'm unsure if its needed this anymore, maybe on AMD. I dont know which ones are those exactly.
Vanilla kernel: the kernel is the core of OS, there is the "vanilla", Apple's original that only run on Apple's similar CPUs, and patched/hacked kernels like "voodoo" that runs on mostly any CPU, they are mandatory to use a hacked one for AMD CPUs.
The vanilla kernel is installed by Leopard, so normally if you dont select any other kernel you will use the vanilla, other kernels need to be installed on purpose.
Generally newer versions have better compatibility with newer hardware (like newer graphic cards, etc.).
Onboard graphic cards may work for install but if they have shared memory you will most likely not get a working accelerated desktop with all the bells and whistles.

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AsRock P45TS | C2D E8200 | GeForce 8600GTS
Mac OS X Snow Leopard 10.6.1 + Windows 7 Ultimate 6.1.7600 + Fedora 11