Here's part of the issue. The slower 4200 drives likely use less power and generate less heat. So the issue may be that they wanted to meet some operating temperature specification (the entire environment of the laptop).
Given it's rather new, I'd chance the change to 60GB.
PS. NO ONE will guarantee the results.
I would like to replace the 20GB 4200rpm hard drive of my two yeard old Fujitsu-Siemens Laptop E7010 (Windows Xp professional) but I am not sure about what would be a good substitute.
I was thinking about a Seagate momentus 60GB 5400rpm, but when I asked the FS support they told that is not certified and they gave a list of pretty old and crap drives that are certified. Has anyone a tip for that?