If you plan to install a larger single primary drive you will first need to partition, format, and install the OS. Once you OSed a new drive you can create large iso type images from the files and folders on the drive. Cloning drives with XP can lead to problems at times. Norton Ghost, Acronis disk imaging, and several other softwares for cloning drives have been available for some time now.
An 80gb drive isn't that large for copying direct onto the new drive temporarily set as a slave to the new primary. If you are running or planning to run XP note that it "prefers" a new and fresh partition to install onto as part of the hardware detection and snapshot of the system as part of the product activation process. Firewire direct transfers would be more reliable then moving large files by way of usb. With two 250gb drives installed here the occasional 180gb of video files is simply copied direct from one to the other when making partition/OS changes.