You can upgrade from 2000 to 2003 or 2003 SBS (unclear about 2003 R2 or 2003 R2 SBS), but NOT to any flavor of 2008 or 2008 R2.
The server can be upgraded to a second processor (provided processor model, stepping match exactly) and up to 16GB of RAM at anytime - whether this is worth the upgrade, depends on your infrastructure and what you need it to do.
I do not believe 2000 CAL's will work for 2003, but I'll let someone more versed in older licenses give you a definitive on that.
Should you update to 2003? What do you hope to accomplish by upgrading? What is it you need?