Maxis has announced the final system requirements for The Sims 2 today, which are the same as told in September. Here they are again for those who forgot:

• Processor (CPU): 600 MHz
• 128 MB of RAM on Windows 98, 2000, or ME, 256MB of RAM on Windows XP
• Graphics: GeForce or Radeon Series or better (older models require a better CPU, about 1.5 GHz)
• Screen Resolution: 800x600 minimum (like with The Sims 1), 1024x768 recommended.

Of course, the better your system, the better the game will run. Although it's not mentioned, it'll probably require at least DirectX 8, or perhaps even 9. Note that these are minimum requirements, and the game may not perform well if you only just meet them. Also, it's quite logical that when you expand the game with downloads, or get bigger families, the performance might decrease. You can check out this page at the official Sims 2 site for more information about the graphics card requirements.