I am running my server: Windows Server 2003 in a Pentium II 400 MHz with 512 MB RAM.
If you already have Windows 2000 in a good working condition, rather than potentially disrupting your stable environment by upgrading to Windows Server 2003, you might want to leave it as it is.
Otherwise, having Windows Server 2003 installed is clearly a better choice for you.
Cheers,
McoreD