I would like to use the A650IS for taking close up and intraoral photos

The camera sells for $1500 at the Calif Dental Assoc. for dental use. No intraoral application but instead used a mirror with macro to take intraoral pics. They said that they did some programing inside the camera but I think what they did was just program the "Custom" botton to do the close up photos.

Any suggestions what I can do so that I don't have to spend that much money and still get as good a results.

For $1500 they must have done something spectacular to this camera, probably put in a completely different lens that is a specialized Macro lens, because $1500 is more than $1000 what the consumer camera is worth. Maybe a different CCD, processor, and lens... who knows... It's hard to tell without seeing the specific camera specs. Anyway, for a fraction of that price you can buy an SLR camera, I'd recommend probably the Canon xTi or xSi for something like this. It is a bit bigger, but I know a lot of dentists use this one, or the new Nikon D300 that came out, with a Macro lens. The body plus a Macro lens may run you around $1000, plus or minus some depending on the deals you get. You can always step down in models to save money, and you should still get a good macro shot. I'd invest in a ring light if you can find one as well, it will probably help with the shots you're taking.

The only problem you have to overcome is the flash. For intraoral pics to turn out good you need the ring flash Aditya D mentioned and there are none available to operate on this camera. If you can find a ring flash that has its own power supply (separate from the camera) AND triggers from the light of the cameras built-in flash you would have a successful device if you can get the flash adjusted to the right level.
Intraoral pics are very difficult.