from
http://www.planetdoom.com/doom3/requirements/
Minimum Specifications
A 1.5-gigahertz Intel Pentium 4 chip or AMD Athlon 1500.
384 megabytes of memory.
Two gigabytes of hard drive space.
An nVidia GeForce 3 graphics card or better; or an ATI Technologies 8500 or better.
Recommended Specifications
2Ghz CPU or AMD 2500+
512 megabytes of memory.
Two gigabytes of hard drive space.
Radeon 9700 or an nVidia GeForce4 ti4600 graphics card or better.
5.1 surround sound audio