Can I ask, what does changing the kernal do? I know about modded roms, but what's the benefit of changing the kernal?
It depends a bit. Some kernels allow some additional features like overclocking/underclocking as an example.
I'm using cm7.2 on my HTC Desire and I flashed Tiamat kernel which allows me to underclock to 128mhz and overclock up to 1.2ghz (native highest speed is 998mhz on my Desire).
Also brings additional governors like smartass v2, which is said to have a better control over cpu demand increasing this way the battery life. Of course if I keep running high cpu tasks will end up spending
more but you get the idea

If you need additional tips about how to's let me know and I might be able to help you

But pretty much you have to get a kernel compatible with your rom. Some kernels are also more stable than others, but that's another story.