I know that a lot of people use Macs here. Hopefully they will get back to you, but I wanted to say that you can always download trial versions of programs and use them for 30 days to see if you like it before you buy it.
Adobe started out with its Photoshop on macs, and the Photoshop Elements is a pretty powerful program for under $100. You can download the trial version here:
www.adobe.com/support/downloads/ . The PSE 4 hasn't been released for the Mac yet due to Mac code issues but PSE 3 should give you the basic idea of the program and if you end up purchasing it, they probably will end up upgrading you to 4 for free or greatly reduced depending on when it does finally come out. I use PSE 3 (on a PC) and it really is a powerful, easy to use program (once you overcome the initial learning curve). I have big boy photoshop too, but I use PSE for 99% of my stuff because its what I'm used to.
But I'm sure others will tell you that their program is the best too! Whatever program you decide, make sure you do the beginner tutorials here, it really does help! Good luck and ask any more questions you have!