How much are you trying to enlarge you image?
Unfortunately, you can never create pixels that would have been there if the photo had been taken at the higher resolution.

You can only approximate. Various techniques do the job better than others. For example the
PS supplied bicubic spline is better than linear interpolation at the expense of increased computation. Fractals are "usually" better than bicubic, again at increased complexity and computation cost. However, they all have their limits.
Having said that, ThomasGeorge seems to have used this program a lot. Check out this thread.
Fractals
I would PM him.
Good Luck,
Catia