2.20. I have >16MB of RAM. Will this cause any problems?

Apart from performance issues, no. FreeBSD 2.X comes with bounce buffers which allow your bus mastering controller access to greater than 16MB. (Note that this should only be required if you are using ISA devices, although one or two broken EISA and VLB devices may need it as well).

Also look at the section on >64M machines if you have that much memory, or if you're using a Compaq or other BIOS that lies about the available memory.