Hi Jim,
You are very far from any limit based on the software. Apple at one point said that Aperture can handle 1,000,000 Images. I run some large Libraries, and so built a test Library of 400,000. Aperture handled it without problem (depending on hardware, loading can take a while, and Faces is very slow). IME, Aperture's resource handling is superb.
Something else is going wrong.
- How are you importing?
- Why didn't you just upgrade iPhoto and Aperture, upgrade the iPhoto Library, and then open it in Aperture?
- What error messages did you receive (quote them)?
- Do you have plenty of free storage space?
- Are iPhoto and Aperture running well otherwise?