bstork,
If the file has come from a scan of a skull I'm guessing that the IGES file is comprised of tens/hundreds of thousands of triangular facets, the number relating to the scan resolution. The file size probably exceed the interface limits and/or the analysis package limits. Also, even if you do get this huge model in, remember that the resulting FE model will be 100-1000 times bigger (but not 100-1000x the IGES file, IGES is an inefficient file format).
I strongly suggest that you put a much smaller resolution model through, say 1000-3000 facets for a skull, do a simple analysis and then review your situation in terms of analysis time and result quality. You will then have a sensible benchmark against which to make decisions.
I strongly disagree with Johnhors that you can buy your way out of trouble in this particular case with a bigger, faster computer, although you may need one anyway to make your life easier.