You might want to just move up to a Mac Pro soon as you can. Not cheap at all, but you will be so much happier - and it should last you for years.
This is EXACTLY, what I just did. Order of progression: Macbook Pro to iMac to Mac Pro.
If you are an amateur working with large and medium format scans, I believe the thing limiting your workflow is RAM and storage, not CPU speed so an older Mac Pro might suit your needs. I say that not because I don't think your images are "worth" a better system ('cause I know some amateurs who can shoot circles around some pros) just that money is big consideration, and since you aren't charging a client for your time you can afford to wait around if it means saving some of your own money.
While, I'm not an Amateur, I decided, I would get my feet wet, by buying an older MP, as the CPU/ Core Increases, do not currently affect my needs. However, RAM & HDD Space are much more important. I ended up buying a 1st Gen, from its original owner. Looks just like the Mac Pro's at the Apple Store.
🙂 When it arrived it only had 2GB of RAM (4X512MB). Now it has 12GB (6X2GB), courtesy of OWC. And it also only had 2X 250GB HDD's in the 1st & 2nd Bays. While I've kept the 250GB in the 1st Bay, for OS purposes, I've added a 64MB/7200 1TB for scratch purposes in Bay 2, and I am linking two additional 1TB HDD's in a Raid 1, via Software Utility, for storage purposes internally, till the files are moved to my NAS Array.
With PS CS3 (25 Images Open), Photo-Mechanic, & Firefox running, I'm only consuming 3551MB RAM. I still have 8732MB at my disposal.
😀 Now it pains me to use my Macbook Pro, for editing.