Here are doom 3's specs, maybe this will help you decide.
id Software (Todd Hollenshead specifically) have finally spoken and have revealed the official minimum requirements these are:
Minimum:
P4 1.5Ghz CPU (or equivalent).
384MB of RAM.
64MB graphics card (see below for chip details).
2GB of free hard drive space.
Lowest supported GPU is a Geforce 4 MX (worse than Geforce 3).
Supported cards:
GF 4 MX.
GF 3.
GF 4.
GF FX (and higher).
Radeon 8500s, 9000s and higher.
PC Gamer were the first to review they themselves recommend:
2Ghz CPU.
512MB RAM.
Radeon 9800.
5-channel sound card and speakers.
For historical purposes only:
From what John Carmack has said the minimum specification for running the basic DOOM 3 engine will be a:
1000Mhz Athlon/Pentium CPU.
256MB of RAM
GeForce 1 / Radeon 7xxx level of graphics card.
'Full impact' based on some estimates (not official):
1400Mhz Athlon / 1800Mhz Pentium 4 or higher speed CPU.
256MB of RAM + again quite dependant on how memory hungry your OS is.
GeForce 3 (original or Ti 500).
Current (June/July 2003) high-end systems should be kicking ass:
Athlon XP 3200+ 3.06Ghz Pentium 4
512MB of RAM
Radeon 9800 Pro or GeForce 5900 Ultra
Source:
http://www.doom3portal.com/info/systemreq.php