Photoshop Elements is a very capable program at a price similar to the PSP you used and has essentialy the same features. One drawback at the moment is that it has not been updated for the Intel Macs but now that CS3 has been released it is thought that Adobe will update PSE for the Mac shortly (don't ask me what shortly means).
Unfortunately, for some strange reason, Adobe does not have a trial version of PSE for Mac although they do for Windows.
Another thought might be to try the 30 day trials of Adobe Lightroom or Apple's Aperture. These are slightly different types of programs that while you can adjust and organize images they do not have selections and layers type of editing features. They both have tutorials available that give good explanations of what they are about. They both are around $300 though.