You don't have to flash a custom kernel if you don't want to. All ROMs already have a kernel included. Some ROMs already have a custom kernel included. You have to read the information in the ROM/Developer's post.
Kernels allow for further customization (like overclocking the phone). Read before you decide whether you want to start flashing kernels because you don't need to if you don't want to. It is pretty easy, though
Here is a very good ICS rom - [ROM] Unofficial CM9 for Supersonic Nightlies - xda-developers
Developer is very good. The only things not working are 4G and FFC.