For now, after getting your iMac, max out the ram. Do some editing. Time how long it takes based on the size of the file your working on. If your happy with how long it takes to process your video after you edit it, then skip the external scratch disk. Its not a necessary item but it does help speed up the process. I would still recommend getting a external drive for back ups. If you can afford a big one, get it (say at least a 500gb). You can partition the drive and set aside around 50 to 100gb for scratch. One thing nice about the mac is if you back up using superduper of CCC, you can boot off the external if your hard drive crashes. I used to render large files while sleeping. The editing bug bit me so upgraded to a mac pro. What kind of camcorder do you have? If you need any tips, I'll be happy to answer your questions. I enjoy editing allot.
Thanks, I've got a Canon HF10 by the way.