Ordering Varchar Column Numerically

Is there a preferred way to order a varchar column numerically (for those
that are numeric), then alphanumerically for all others?

I've tried:

ORDER BY CASE WHEN IsNumeric(<column_name>) = 1 THEN CONVERT(Float,
<column_name>) ELSE 999999999 END;

and

ORDER BY CASE WHEN IsNumeric(<column_name>) = 1 THEN 0 ELSE 1 END, CASE WHEN
IsNumeric(<column_name>) = 1 THEN CONVERT(Float, <column_name>) ELSE 999999
END, <column_name>;

Neither of these however give the desired results.

Any ideas?

Thanks,
Frank