How would I get it in my iMac?
Good luck with this mate!
You can also sell your 512mb sticks on ebay, and make back some of the cost of upgrading.
What kind of computing do you do? As MaDDoG said, if you're just doing the basics, you won't need more than a gig. Photo / video editing, webpage building, or doing music in garageband will require more rams. Your cat should be able to upgrade it for you. If you don't have a cat... you're screwed.