I'll try and sum things up by comparing them to what you already are familiar with - this is not exhaustive, just some of the major trends and variations listed.
Woods - the vast majority are mahogany/maple top like your RG8570 which give a warm tone. Your alder RG8320 is a rare exception to that rule. Also some of the early models had ash bodies e.g. RG1302 which give a bright tone with lots of attack and clarity (or less warmth and depth, depends on your point of view and preference) Some of the earlier models were made of 3-piece instead of single piece bodies. Also bear in mind every piece of wood sounds different, so it's luck of the draw what tone you'll get from each individual guitar. Occasionally rarer woods are used e.g Koa tops.
Years - generally higher specs added over time e.g. Jem-style input jack, rounded fret edges, J-custom AANJ instead of Prestige style
AANJ neck joint, velvetouch
neck finish, recessed vol & tone controls, more abalone on the neck vines. Trems have moved over time from Lo-Pro to Edge Pro to Edge Zero for 2008 - personal preference which you like best.
Necks - last year's e.g your RG8320 were the thinnest so far. Your RG8570 feels about the thickest I've played. The older JCs with the Ultra profile are fairly beefy compared with your RG8320 but not as beefy as your RG8570. Occasional neck-throughs released, but usually bolt-ons.
For a good run-down of the ranges available since 1996 take a look at
http://jcustom.web.fc2.com/index.html, I find that a handy reference - just click on the model names on the left. Also
http://www.jemsite.com/jcustom/index.htm has good details, especially on the early models.
I hope that helps. Good luck in building your collection. J-customs own :-)
