iPhoto as you know will put them into those folders based on the capture date. If you import the images into iPhoto folder by folder then the "Roll" title will be the same as your folder title.
You might want to rename the subfolders something like this: Main folder-subfolder, i.e. "Family:2003-1-01 New Year's". That will identify each sub folder as to it's next level category. You can then create albums in iPhoto and further organize the photos. REMEMBER, however, that your files will be copied into iPhoto's crazy folder structure.
I would copy your folders of image files onto your new computer intact and leave intact as a backup. I recommend you frequent the Apple iPhoto discussion forum answer any questions that may arise. When it works, it's terrific. But can give you problems for various reasons. I've never had a problem and have 5 separate libraries with about 6,000+ image files. However, I also use iView MediaPro as main image management program and iPhoto as a complementary.
I guess the message I'm trying to convey, but not very concisely, is to maintain backups. The Apple iPhoto Discussion forum:
http://discussions.info.apple.com/ is a great place to learn about tricks and solutions to problems. It's a must URL if you're serious about iPhoto.
Hope this has been of some help. Good luck.