When to Rebuild indexes in SQL server 2000

Hi, I am a new into sql field. I want to know that when we have to decide to rebuild indexes. What kind of cammands i have to run and how to analyze thier result before going to rebuild index.I ran DBCC SHOWCONTIG (Tablename) anf got the following results.
TABLE level scan performed.
- Pages Scanned................................: 3
- Extents Scanned..............................: 2
- Extent Switches..............................: 1
- Avg. Pages per Extent........................: 1.5
- Scan Density [Best Count:Actual Count].......: 50.00% [1:2]
- Logical Scan Fragmentation ..................: 0.00%
- Extent Scan Fragmentation ...................: 50.00%
- Avg. Bytes Free per Page.....................: 246.7
- Avg. Page Density (full).....................: 39.95%
I know that the DBCC SHOWCONTIG command provides several measurements, the key one is Scan Density. This figure should be as close to 100% as possible. If the scan density is less than 75%, then you may want to reindex the tables in your database and in my case its 50%. I can not go to rebuild my index only on this concolusion. Can some one tell whatelse and where else i have to look to know that these indexes needs to be rebuild or refrag.
Thanks in Advance.