There are more than 10 changes or improvements in this beta, including improved graphics performance for games and 3D apps (via OpenGL 2.1), improved USB and Boot Camp support and lots of UI improvements. You can get the full run-down here.
The increase in virtual disk size is significant. Prior to this release, the max was 950GB. The update also bumps up parallel processing performance with support for 8-way SMP. Anyone testing the beta is encouraged to share their experiences in the beta forums.
Earlier this week, MacTech posted a video of Parallels out-performing VMWare Fusion in a graphics-intensive, side-by-side comparison. Perhaps the changes in this beta will address those shortcomings (though I'd just boot into Windows via Boot Camp if running a 3D game).
Fusion costs US$79.99 as a download or $99.99 for the app plus upgrades for the next 12 months. It requires an Intel Mac, at least 1GB of RAM (2GB RAM recommended), 700MB free disk space for VMware Fusion and at least 5GB for each virtual machine; it runs on Mac OS X 10.5.8 or later; Mac OS X 10.6 or later. If you want to try out the beta, you will get an expiring serial number; you can also use your existing Fusion 3 serial if you already own the product.