Create Index with a WHERE clause
New IDs will always INSERT into the table, not the index. Your proposed INDEX would merely provide a more efficient lookup for some ID values.

Some thoughts though

The index may not be used too: it would have to cover all columns in the table. Or searches would have to return just a few rows so the key lookup needed (for a non-covering index). For more, see "Using Covering Indexes to Improve Query Performance".

As it stands, it may help, it may not.

Deleting or archiving old records can be made less painful too, which is probably the better solution. Example (SO): https://stackoverflow.com/a/2126482/27535