Oracle capacity

Iam running oracle8i on hpux 11. This is test box with 4 CPU's and 8Gig of memory. The databse already has 5 databses. I have been task to see if we can run anymore databases on this box ( ie we will like to add 4 more databses) or buy another box. These databases will have copies of production data which is about 6gig in size at the moment.We are not worried about space but can anyone advice me on how what issues to look for or how to determine if these extra databases can be created on this box without affecting perfomance

Thanks in advance

Total available Memory - (Sum of instances for esisting DBS + Sum of estimate of the instance of DBs you're planning to create + additional space in the memory required torun additional process for OS. )