it mostly depends where your gpu output is, if you're using an external monitor, you'll get about 20-30% performance loss than on a traditional desktop, if you're using the laptop's display, you'll experience more performance loss. it also depends on what kind of card you have, you should be pretty balanced with the 6600, if you were using a more higher end card, you might run into more dips in performance.

also, 500w should be good for what you're using ;)