Each bitmap or Photoshop image you scan will be 20 to 25 Megabytes in size, per layer.
Images should not be converted to JPEG or PNG or any lossy compression type of file until all corrections have been completed. You can however save space by using ZIP compression on a file to save disk space.
No matter how you try to save space you will almost certainly run out of hard disk space, with only one exception - he who backs up his images to CD and then deletes the hard disk file.
Therefore, unless you plan to have a CD writer, you will need a hard disk as big as you can afford. Every modern computer can handle four IDE hard drives. If you have a SCSI controller then you can have many more SCSI drives in addition to four IDE.
Alternatively, if you additionally have a cd-writer and a DVD player connected to two out of the four IDE ports on the motherboard, then consider adding an additional hard disk drive controller card. Adding a hard disk drive may be more complicated if you want to move applications from say disk C: to D:, because of the registry. You will probably have to re-install these applications, unless you use a utility like Partition Magic which can modify registry settings. I use Partition Magic frequently!
Because there is always a risk of formatting the wrong hard disk, or a power failure when transferring data from one disk to another, modifying an installation can be a very worrying time for the in-experienced, that's why I always recommend you buy as big a hard drive as possible. For example, during my recent system upgrade my SCSI boot disk moved from C: to F: whenever I attached a fourth IDE disk! I think it objected to a disk with non-DMA support on a controller with another disk having DMA support, but why it changed the SCSI boot disk I'll never know.
Maintain your hard disks by removing redundant temporary files and duplicate copies of applications. Use of McAfee Nuts and Bolts or Norton Utilities can help to keep the hard disk tidy.
By far the biggest help is keeping everything neatly in directories or folders.
In general if anything goes pear-shaped it's always with the operating system. By keeping only the Windows operating system on partition C: it gives you a 95% chance of using Norton Ghost to create an image copy sufficiently small enough to write this to a single CD. Although you can span across multiple disks if required. Recently I have recovered from several serious system crashes (preventing me to boot into safe mode) many times using the Ghost image. Thus it has saved me several weeks of work re-installing everything. Image copies of complete drives can also recover partition information, system and hidden files and are much better from straight file or backup copies.
Keeping data files within the My Documents folder, with sub-folders for say Word, PowerPoint, Publisher, etc., makes it a lot easier to backup all important documents and reduces the risk of missing something important. Create a Photo or Images folder for your scanned and manipulated, as the total capacity of these files will require writing them to a separate CD than all the other My Documents.
Frequently hard disks fail (generally the File Allocation Table or FAT gets messed up) when there is insufficient hard disk space to perform a write operation to the disk. This can occur by simply sending a file to print. I have known a print command to fail with 75Mb of hard disk space free, so I now recommend keeping at least 100Mb spare.