I just upgraded my Dell laptop from 120 gb HD to 320 gb. Tiger Direct has Seagate 320 gb 2.5" drives for $89, and I got the Apricorn EZGig II USB II upgrade package, without hard drive, for about $40, direct from Apricorn.
You put the new drive in the EZGig enclosure, plug it into the laptop via the supplied USB cable, and run the software from the EZGig CD. It took about 2.5 hours to clone the 120 gb drive, which had about 100 gb of data on it. There were a few false starts, but it eventually worked. This was with Windows Vista Business Edition.
One nice thing about the EZGig hardware is that it also serves as an external drive caddy, once you've swapped drives. It works with SATA or PATA 2.5" drives.
Long live Bill Gates!!!