There is no short answer for these questions.
Here are some documents you might want to have a look at:
Oracle® Database Upgrade Guide 10g Release 2 (10.2)
Forms 6i: Deploying Forms Applications to the Web with Oracle Forms Server
As for the memory limit, that depends on the hardware and the operating system. If you have the standard edition, I think you can’t get it to use more than 4 GB (even with PAE enabled).
Have a look at this Microsoft document: Memory Limits for Windows Releases