There are two levels of recommendations for Windows Vista, those dubbed "Vista-Capable" (supporting the minimum requirements) and those dubbed "Premium-Ready" (recommended).

Minimum Requirements (Vista-Capable PCs):

• 800 MHz Intel-compatible processor
• 512MB of RAM
• DirectX 9.0-Capable Graphics Processor
• 20GB HD

Recommended Requirements (Premium-Ready PCs), that will also allow you to enjoy the new Windows Aero Interface:

• 1 GHz Intel-compatible processor
• 1GB RAM. Vista 32-bit will work ideally at 1GB, and minimum 512. 2GB is the ideal configuration for 64-bit Vista However, since 64-bit is handling data chunks that are double the size, you'll need double the memory, hence the 2GB.
• DirectX 9.0-Capable Graphics Processor, with 128MB graphics memory. (64MB of graphics memory to support a single monitor less than 1,310,720 pixels [no more than 1440x900]; 128MB of graphics memory to support a single monitor at resolutions from 1,310,720 to 2,304,000 pixels [no more than 1920x1200]; 256MB of graphics memory to support a single monitor at resolutions higher than 2,304,000 pixels [more than 1920x1200]).
• 40GB HD with at least 15GB "free space"