Which objects to collect statistics? (Oracle 9i)

ALL, for sure is the best answer.

But a client came with the following issue: he was sure that the lack of updated statistics were hurting the performance, and he didn´t have time to collect ALL.

Then I told that he should collect statistics on the smallers objects cause if an object use to have 1.000.000 rows and now have 1.100.000,  probably the otimizer will keep the same execution plan, but for instance and object use to have 2 rows and now have 200, it can change dramatically the performance.

How to split SGA DB_CACHE between production and DEV DB

You can put the DEV instance objects on recycle area, them it won´t fight against the production DB.