Video card must be 128 MB or more with support for Pixel Shader 2.0b (ATI Radeon X800 / NVIDIA GeForce 7600 or better).
You can often find a bundle containing both Portal and Portal 2 for a price only slightly higher than a single game. portal-2-pc-game-free-download-full-version
Portal 2 is well-optimized and runs on a wide variety of hardware: Windows 7 / Vista / XP Processor: 3.0 GHz P4, Dual Core 2.0 (or higher) Memory: 2 GB RAM Video card must be 128 MB or more