To use Mac OS X 10.4 Tiger, your Macintosh needs:
A PowerPC G3, G4, or G5 processor
Built-in FireWire
At least 256 MB of RAM [note that without more than 256MB of RAM, your computer will run much slower]
DVD drive (DVD-ROM), Combo (CD-RW/DVD-ROM) or SuperDrive (DVD-R) for installation
At least 3 GB of free disk space; 4 GB if you install the XCode 2 Developer Tools