Seriously, Ribcraft have a good rep and the 4.8 and 5.85 seem to hold their value well (sorry but I don't know too much about the 640).
I had a go on a 585 with a Suzuki 140 at the boat show and I really liked it, the outboard was very quite.
Ribcraft have always been very helpful when I have called them, so give them a call if you need to.
Last of all Ribcraft are currently selling the Olympic boats off, no 640's but some 585's and I think 750's?