In upcoming games using the Steam engine and even possibly some older games such as Half Life 2, you'll see smoke which not only drifts around the room but hits the roof and floats out the door in a true-to-life manor. You'll see individual rain drops which put out fire one by one. You'll see advanced AI which delivers more advanced computer enemies. Since the game has more processing power through the additional supported cores, computer enemies will be smarter and even be able to perform tactical analysis against you - for instance, working out the best place to hide because it has the processing power to work out where you are in the world and even where you might go to next. Maybe sometime in the future the US government will contact Valve to create some practice tactical military software for them?

Dual Core improved frame rate, Quad Core will add new experiences to the game such as life-like realism. Valve are bored (somewhat) with the GPU now - as they said, they can already create a model on the GPU to look like a real person but that model cannot act like a real person, yet. Now they want to focus on the CPU and create extreme realism. People ask when Half Life 3 will be ready but as Valve correctly state, the changes that they've made (or are currently making) are as good as a new version of the game.

These Multi-Core enhancements will be gradually added into the Steam engine and existing and future games using the engine, such as Episode 2 sometime early next year of Q1. There will be no single patch / update with the changes - they will be added over time.