When I first got an iMac about 4 or 5 years ago the only software I purchased was MS Office (Word, Excel, Powerpoint) and Quicktime Pro. You can get the student/home edition of MS Office for about $100. There were some deals right after Thanksgiving that had it for $80 but not sure iof any are available now.
I used the iLife software (iPhoto, iMovie, iDVD, Garage Band) for photos and video for over a year or so before I purchased anything else. The software is quite powerful, easy to use and produces great results so you might hold off getting other software until you try out what the Mac has pre-loaded. I have MacBook Pro and I still use all of the iLife software on that machine.
I have over 15,000 photos and wanted to store some of them in different locations and catalog them differently than IPhoto supports so I purchased Apertrure. IPhoto does allow you to do some cleanup of photos and supports Raw formats (at least Cannon's). I wanted to do some graphics work for things like cruise magnets and T-Shirt transfers so I ended up purchasing Photoshop Elements which also supports some different photo cleanup.
I made some fairly elaborate videos with iMovie but ended up purchasing Final Cut Express to do some more complex things with audio than IMovie could support. I still use GarageBand for most of my audio cutting and iDVD to burn my videos.