Greetings,
there are a few things to consider when you build a very large single raster dataset. First off, you were able to compress your data in the geodatabase with jpeg2000 compression. If you plan on writing a IMG file then you won't be able to compress with jpeg2000 and your resulting image will be much larger. You can write a very large jpeg2000 file as well to keep the compression.
Another problem is the amount of time. As you already noticed with the loading into the geodatabase, writing such large files takes a lot of time. If you want to do this you probably want to practice on a couple subsets first, making sure that you get your mosaic method correct. Mosaicking will take a long time and you will lose the additional information of overlapped areas.
Something else you can try (though it probably won't increase your display speed) is to create an unmanaged raster catalog. This is an easy way to manage your data but you'll always have the raw data on disk that you can still use. These create quite quickly and you can play with that. The unmanaged raster catalog is also the suggested workflow to mosaic to a single raster dataset (not in the geodatabase).
If you do have ArcGIS 10, I would suggest trying the new mosaic dataset data model. This is similar to an unmanaged raster catalog but has many additional benefits such as overviews (like pyramids for your collection of imagery) that will increase your display speed.
Hope this helps.
Robert
