Ok, your first stop is the dicumentation which you should find on your 9i cds.

Alternatively http://www.oracle.com/technology for all docs, including the 10G stuff.

Early versions (9201) of oem did have memory leak issues, especially on unix. We still run oem 9201 on windows NT(!) and 2003 without issues. Some early 9i agents also cause memory leak issues.

Depends on how many nodes/targets you want to monitor, whether to put it on a separate server (if its more than a few I recommend you do). Whatever you do, make sure you have the recommended resources (RAM, CPU) available (see the docs) on the server.

Consider 10g Grid control as well, although release 1 is a little buggy, its still useable (release 2 out later this year) and has some really nice features. Yes 10G will quite happily monitor 9i databases, you just need to install a 10G agent on the target server(s).

Hope this helps

Bazza