Here are some test results: (made on my PC, not on the real site of course)
For a current comments size of 44.9 MB,
- dropping currently archived comments: gain = 1.1 MB
- dropping maximum overdrive comments: gain = 0.5 MB
- dropping explorer4x4 & rockerbj comments: gain = 1 MB
- dropping comments that are more than 1 year old: gain = 8.5 MB (only!)
- dropping comments that are more than 6 months old: gain = 20.8 MB
- dropping comments containing imageshack picture links: gain = 13.6 MB

Note that combining 1st and 3rd one does not allow to gain 2.1 MB, as most of the archived comments are actually from Explorer4x4