OK, some 'myths' to bust here:
A) I wouldn't worry purchasing from a PowerSeller as other have mentioned. I have rarely had issues buying from people with great feedback on eBay. The goal of the feedback system is to be able to discern who is a great buyer/seller from a terrible one.
B) I purchased my MacBook Pro Core Duo 2.16 Ghz brand new 2 years ago. Mine is still under AppleCare. I personally would not purchase a the 1.83 Ghz Core Duo due to the 128 VRAM. Some 2.0 CDs and ALL 2.16 CDs had a 256 ATI card.
I believe the newer Core Duo builds were of better quality then the older ones. When I purchased my machine, I had 3 DOAs from the Apple Store. The manager could not believe my luck (shows the quality of some of these older machines). Mine has been PERFECT once I got a working one. No dead pixels, no hum (as some owners -- especially 1.83s have expressed), not running extremely warm. Granted, I have not used a Core 2 Duo.
C) This machine is plenty fast for today's standards. Even though there is a 2 GIG RAM limit, this machine performs flawlessly and fast. I upgraded my small 100 GB HD to a whopping 500 GB (obviously I am VERY happy with my machine). I do not feel technology has progressed enough to upgrade from a Core Duo to one of the newer unibodies or Core 2 Duos.
Basically, what I am trying to say is you cannot go wrong with a Core Duo. It will run Leopard perfectly, play most games at excellent settings, run Vista via Bootcamp or Parallels/VMWare, etc. These machines are plenty fast. The only reason I would go for a Core 2 Duo is if you are a heavy user of Parallels/VMWare. I find the 2 GBs to spread thin between Leopard and Vista Ultimate side-by-side. That is why I prefer to use BootCamp.
Let me know if you have any specific questions about Core Duos.
EDIT/Forgot to add: my only AppleCare issue was getting a replacement battery under the battery exchange program. Many of the Core Du knowledagble os had bad batteries which, if the owner was at all, could get switched for a new battery. I believe the program is still running on a case-by-case basis.