The 500MHz models all had 66MHz busses, and all were PPC 750cx. These were the original iceBooks.
The next revision offered a choice: a 500MHz processor on a 66MHz bus, or a 600MHz processor (both 750cx (256k L2 cache)) on a 100MHz bus. Both had the same 8MB RAGE 128 Mobility graphics chip and other onboard stuff, but the 600MHz model had different drive options.
The revision after that changed things: they went to a 600MHz-minimum PPC 750fx (512k L2), with a 16MB Mobility RADEON video card. This revision's board is noticeably different from the previous versions, and thus may not fit properly into your model (one of the first two versions) without heavy modifications. These models are also prone to the GPU problem, whereas the first two didn't really have any issues with it.
If you're going to do a logic board swap, it's best to stick with the 600MHz 750cx board - it's going to be a challenge enough just to replace the board (you hafta strip the ENTIRE computer to replace the board, much like the original iBook), let alone performing any potential modifications and dealing with any future failures of one of the later boards' GPU. Plus, you'll notice enough of a difference between a 66 and 100MHz bus and an extra 100MHz in processor frequency to make the swap worth the effort.