Are there any restrictions on how I divide the disk up?

Yes. You must make sure that your root partition is below 1024 cylinders so the BIOS can boot the kernel from it. (Note that this is a limitation in the PC's BIOS, not FreeBSD).

For a SCSI drive, this will normally imply that the root partition will be in the first 1024MB (or in the first 4096MB if extended translation is turned on - see previous question). For IDE, the corresponding figure is 504MB.