After consulting several posters on both this forum and the German hifidelio forum (they were extremely helpful there, even when I posted in English), I took the plunge and upgraded to a 500 GB drive.
I used a Western Digital IDE WD5000AAKB ( 500GB 3.5 7200RPM 16MB Buffer).
I checked the specs of my original HD (Samsung SP 160N4) and this one seemed to have similar electrical demands (especially during spinup) and heat production.
There was only one glitch during the switch: my Olive/hifidelio refused to read the "hdreset" disc I burned. I used the alternate method, hdreset over telnet: see file at: www.8ung.at/ascherrer/hdreset.tar.gz
The German forum has instructions in the "Modifications" section.
So far, so good. It's only been a day, but I'm reimporting all my CD's in uncompressed formats, and so far about 60 CD's in a row with no performance problems.