Estimate database usage - Oracle

One question always haunts me a bit, "Could you provide the database machine requirement?".
Here's one method I often use to estimate the size for Oracle database:
1.  SELECT SUM(BYTES) FROM (
select SEGMENT_NAME, SUM(BYTES)/(1024*1024) BYTES from user_extents
where segment_type = 'TABLE'
and segment_name like 'ACT_%'
GROUP BY SEGMENT_NAME);

select SEGMENT_NAME, SUM(BYTES)/(1024*1024) BYTES from user_extents
where segment_type = 'TABLE'
and segment_name like 'ACT_%'
GROUP BY SEGMENT_NAME;