Oracle 8.1.5 will setup on Windows 2000 BUT you will encounter weired behavior from it like a sudden hangups from SQL+ SO I recommend 8.1.6 or 8.1.7 , Both of them work very very smooth on Windows 2000 .

You need to have ( if running the database only with no application server for instance on the server ) 256 M ram + 1 G of NTFS hard disk space for Oracle itself full featured installation.

I am recommending 8.1.7 over 8.1.6 because it has Apache webserver built it thus makes you able to run Enterprise manager and SQL + from any machine on the network via Browser softwrare