From what I know/understand/remember (three very different things, and sometimes it all blends into one thing which is what I think I know) is that the model designation goes like this:

85-86 = ND5
87-92 = NF5
93-00 = NX5
01-07 = NXA

I would recommend you stick to the NX5 or NXA models, and unless it comes with a really good spares kit, get an NF5. I would not (unless for collecting reasons) get an ND5 for a track day bike (just too old and outdated to be worth it).

The NXA (01-07) model is obviosly the newest out there, and it seems also equates to the most expensive to buy trick bits for. Hmm, who would have thought that when I bought my 01?

The NX5 you can make just as trick and competitive as the NXA, plus they are much cheaper overall to buy. You can still get parts a plenty and they spares all run around the same as the NXA stuff. Check out Redline Cycle imports (Craig the owner is a great guy and get's me most my standard RS250 parts in a couple days). His Website is http://redline-ci.com/. Also Rising Sun Cycles (http://www.rscycles.com/) has done pretty good for me getting cool bits pretty quickly. Plus both these place sponsor/support USGPRU racing series

Barber was a fun weekend. It's a very technical track and after my first session I wanted to crawl up in the trailer and go to bed. Took me my second session to figure out the blind cresting turns that cover the entire track. And damn, talk about a physically hard track. I was beat by the second day. Good thing my RS was needing a new top end, I had an excuse to call it good part way through the afternoon
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Charles Gallant