Things like surfing the web and listening to music shouldn't effect (affect?) your PPD much. When playing games, the PPD will drop down quite a bit, and your game may lag. I typically will pause the gpu client while gaming. However, the cpu client will typically put most of its resources into the game, so you don't really need to worry about that.
encoding video can be pretty demanding, and is more cpu based, so a similar situation to gaming will happen.
photoshop will be more like typical day to day things, so you should be fine.
I beat Z to it!!! Edited by hoth17 - 12/22/10 at 6:41pm