You might find that searching for the min. system requirements may bring different results due to personal preferences and such. That's fine, but I'm listing what's on EA's site.
MrsFlynn
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The Sims 3 Pets Minimum System Requirements
http://help.ea.com/en/article/the-sims-3-pets-minimum-system-requirements/PC System Requirements
OS: XP SP2 / Vista SP1 / Windows 7
CPU: For XP 2.0 GHz P4 processor or equivalent; For Windows Vista and Windows 7 2.4 GHz P4 processor or equivalent
RAM: For XP 1 GB RAM / Windows Vista and Windows 7 1.5 GB RAM*
DISC DRIVE: DVD drive
Hard Drive: At least 3.5 GB** of hard drive space with at least 1 GB additional space for custom content and saved games.
Video: 128 MB Video Card with support for Pixel Shader 2.0
* For NVIDIA ION™ computers, the game requires at least 2.0 GB RAM.
** 9.6 GB of hard drive space if installing with The Sims 3.
Supported Video Cards: Nvidia GeForce 6 series or better, and all NVidia G, GT, GTS, and GTX video cards. ATI Radeon™ series card 9500 series or better, and all ATI X, X1, and HD video cards; Intel® Graphics Media Accelerator (GMA): GMA 3-Series, GMA 4-Series
Please note that the GeForce 6100 and 7100 cards are not supported.