IF you bought a full version of 2005, and not an upgrade, then uninstalling the 2003 before putting on 2005 should not hurt. And is what i would do.
If you got an upgrade, it will probably need to 'see' the old version on the PC before allowing the upgrade.
But i guess the full version, so i would uninstall the 2003 first. just remember not to go online till the new version installed and running right, as you will be unprotected.