buffer hit ratio% negative, rollback wrap

[Kumud]

I install sportlight (quest software ) , oracle management server performance dignostic.
I am trying to monitor my production database and found
buffer hit%;
Sportlight shows 0% to 50%
(Oem)Performance manager- 81%
(Oem)memory cache hits , buffer cache hit% negative.

My database has some serious problems, Now can I get correct buffer hit%.

If you do not specify optimal size in rollback segment what will happen ? What is the meaning of rollback wrap, undo header ? Does it effact degrad performance. My rollback shrink is 0.

Please help