Configuring load balanced PWA site

1. Install and configure a WSS Farm on ServerA
2. Add a second server (ServerB) to the WSS Farm created above
3. Configure the load balancing using Windows LB for instance (or using a hardware device)
4. Note the Virtual IP address configured in step 3
5. Creat a new Web Application and under Load Balanced URL enter the adress from Step 4.
6. Create Site Collection/SSP/PWA ... 
7. That's it....

If you change your mind about the load balanced URL, you can modify it afterward in the WSS Central Admin site: OperationsàAlternate Access Mapping link

For more information on Network Load Balancing and Sharepoint please check this excellent post from Joe Oleson: NLB (Network Load Balancing) and SharePoint... Troubleshooting and Configuration tips