Edonkey 2000 is a huge CPU hog. aMule used to be pretty light on your CPU but it is somewhat heavier now since 2.1.0. My guess is, though, that it's still less resource hungry than the official edonkey client.
If I were you, I would just give it a shot: turn off edonkey2000, download aMule, throw in a number of files you want to download, and run it a few hours to see how much of your CPU it takes. I would actually be quite interested in your results, so if you could post them here that would be great.
If it turns out that aMule is still too heavy on your CPU you can try mldonkey (now mlnet). It's missing a number of aMule's features but it's the king of low CPU usage. When I was running it a few years ago on my G4 867mhz it usually used about 1-2% of my CPU.