Here is a link to one of the custom rom threads. I'm not saying this is the rom you should use, although the author is quite skilled and I'm sure it's great... I'm just trying to give you an example.
http://forum.ppcgeeks.com/showthread.php?t=60908
You can see from the title of the thread that his roms are based on the 21038 and 21193 sys versions. Also, he has screenshots and very detailed info about what you get.
Most of the custom rom threads give you similar info, which is, in fact, all the additional info you should need. It's all already out there for you. Get to reading, and have fun.