One option you always have is buying an X or Droid 2 off of ebay and using it for about 6 months, then getting the dual cores that are out. I would be REALLY wary of buying an X/Dinc/Droid2 on a 2 year contract. These phones are already 9 months old...in 2 more years these will be unthinkable IMO. This is the reason that I always get the newest and best in smartphones. If I am going to keep this thing for 18-24 months, why wouldn't you spend the extra 100-150 to get a large upgrade in functionality and satisfaction?
So if it's an option, buy an OG droid maybe for 200 dollars, get a feel for android, and by the time you are a pro with it, a new dual core phone will be out!