load balancer question

I was having a discussion about scaling a php-based app. Most of the sites I've worked on have easily fit into a single server model. I"m looking at a site that will need to scale beyond that. The anticipated bottleneck will be TCP connections and database performance. I'm looking at load balancers and was wondering if people had experience with one that they felt was up to task. We do use sessions and I would not like to go to a saving session data on another box or in the database. I would prefer for the session identifier cookie to determine which box the user goes to. thanks, jonathan