Well, the sync ranges shouldn't case you any trouble, if your new monitor isn't older than your current one, since 21-inchers are expensive kit, usually with good specs (older ones, though, are often very limited in their resolution ranges, since they tended to be used with a particular card or machine).
Your safest bet, though, if it's a semi-normal VGA monitor, is to either see if it has already got a profile in the admittedly small CGX monitor database (there are also additional profiles on Aminet and the CGX homepage). If not, use the VESA modes (assuming it's CGX 4 you're using), since these tend to be both well-supported and well-aligned on most monitors.
