In terms of performance and hardware needs, Premiere Elements and Premiere Pro are about the same. So the issue isn't that Elements is that much "lighter".
Also, when deciding between the Pro and Elements versions (whether it be Photoshop or Premiere), the rule of thumb is that, if you don't understand the differences, you probably won't need the pro version.
Aside from some high-level professional tools, Premiere Pro includes embedded timelines (that's the ability to work on one segment of a project at a time, while combining them together on another timeline), the ability to run several video tracks at once and select which displays (great for cutting between shots of different angles of the same scene) and a bit more tolerance for non-standard video and non-standard frame ratest, etc.
But, in terms of quality of the results and the basic editing tools, the two programs are identical. In fact, many semi-pro videomakers get along just fine with Elements.
That's the good news.
The bad news is that AVCHD video is bear. It's challenging to work with and sluggish to edit. In fact, most people who are working with it successfully (and aren't going nuts doing it) are using a quad-core processor and 4 gigs of RAM. It really is a camcorder technology that challenges current computer technology.
In other words, your challenge isn't between working with Elements or Pro but whether your system can edit AVCHD with either.
As the many posts to the CS4 forum can quickly attest, AVCHD is computer dependent. Many on the CS4 forum feel strongly that only dual-i7's will effectively do it. I've seen many, who do get by with a single Quad. It depends on what one is trying to do with that material, and what they can tolerate.
I also agree with you about CS4 vs PrE. The biggest differences are the GUI (I really like the one in PrPro better, but came from that progarm), and the ability to do several "mini-Projects," the Sequences, at one time. However, with the latter, the load on the computer goes up, as the Project becomes more involved.
As for the two programs' footprint, PrE might get a touch of an edge, but that is because of the inability to Nest, and do multiple Sequences. Still, Adobe aimed PrE for users, who will likely not have a robust workstation, so I feel that it does some things with slightly greater efficiency, all other aspects being equal. Still, no NLE program is that easy on a computer - any computer. These will tax a computer more than almost anything, with RT 3D being one exception. Howver, that will usually tax the GPU much more, and an NLE is really light on the GPU.