The camera has a buffer to store the picture before it writes into the memory card, so you can still meet the 4.5fps. Using the class 6 will of course help because you can clear up the buffer faster...
All SD memory cards indicated above can be used regardless of their transfer speed.
1 If the memory card will be used with a card reader or other device, the device must support 2GB SD memory cards.
Other brands and capacities of cards may work, but Nikon cannot guarantee their operation. Check with the manufacturer of the third-party card for compatibility information. Nikon recommends keeping one of the approved cards available for troubleshooting.