Most Windows 7 users will be running the 64-bit version which I also strongly recommend. When you install IE9, both the 32-bit and 64-bit versions install automatically. You'll likely want to run the 32-bit version for the best performance. The 64-bit will say "(64-bit)" after the name so you'll want to run the one that just says "Internet Explorer."
Key features of IE9:
- Improved standards support
- Improved, minimized user interface with more space for the web site you're browsing
- Improved download manager
- Integration with Windows 7 (e.g. Snap, Pinned Sites, Jump Lists, and thumbnail preview controls)