Alexander, you probably downloaded the full Eclipse SDK, which has the whole Java IDE and all the source code and plug-in developer’s documentation included. And download size doesn’t indicate memory size (given source, doc, etc.). A PHP-only IDE based on Eclipse will be much more lightweight by any measure. While PPC-based Macs are not speed demons (I own one so I should know) I know plenty of people who run Eclipse (the full Java IDE) on it and they seem reasonably happy with it.

Gonzalo, even a fully loaded Eclipse should run way faster on your setup. Feel free to post something over on one of the EclipseZone forums about your setup and experience and somebody should be able to help you find out what’s going on.