Lexar is now selling a CompactFlash memory card rated at 300x speed -- meaning it is 300x faster than the speed of a standard 1x CD drive (150KB/sec). In theory, this card moves 45MB of data per second.
Is it worthwhile paying extra for this theoretical speed? The short answer is, No. You can read the long answer at Imaging Resource: Testing the Lexar 8GB 300x Card.
On most cameras and computer equipment, the read and write speeds differ little from the 60x card that was also tested. Only when the tester used it with extremely fast computers and very high end cameras did the card fulfill its promise.
Save a few bucks; buy a slower card.