Extreme difference in 2 laptops wifi speeds

Hello,
I have a Linksys WRT150N router hooked up to cable (Broadband). On the desktop (XP)using the LAN, the computer download speed and upload speed are as follows: (15000kbps DL & 2900kbps UL)
On one laptop #1 (XP) using an external Linksys N card hooked thru the USB port, the download speed is about 600kbps DL & 2500kbps UL.
On Laptop #2 (Vista) using an internal "G" wireless card the speed is almost as fast as the desktop.

My question is this: Laptops setting side by side why should there be that much difference?

Thanks
Denver